Thermal transfer printing

ABSTRACT

A printing assembly for thermal transfer printing is disclosed. The assembly comprises at least one first printing system comprising a transfer member having an imaging surface on the front side, a coating station at which a monolayer of thermoplastic particles is applied to the imaging surface, an imaging station at which electromagnetic radiation (EM) is applied, optionally via the rear side of the transfer member, to selected regions of the imaging surface to render the particles coating the selected regions tacky, a transfer station at which only the regions of the particles coating that have been rendered tacky are transferred to a substrate to form an adhesive image; and at least one more downstream printing system. The transfer member includes on its front side an EM radiation absorbing layer, the imaging surface being formed on, or as part of, the absorbing layer, and on its rear side a body which can optionally be transparent to EM radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.17/386,383 filed on Jul. 27, 2021, which is a divisional of U.S. patentapplication Ser. No. 16/425,559, filed on May 29, 2019, which is in turna continuation-in-part of PCT/IB2017/057535, filed on Nov. 30, 2017,which claims Paris Convention priority from PCT/IB2016/057226, filed onNov. 30, 2016. The contents of all the above applications areincorporated by reference in their entirety as if fully set forthherein.

FIELD

The present disclosure relates to a method and system for printing on asurface of a substrate with a film of a thermoplastic material.

BACKGROUND

The present disclosure is a development of the teachings ofPCT/IB2016/053139 to the same Applicant which was published asWO2016/189512 on 1 Dec. 2016 and has a priority date of 27 May 2015. Toavoid unnecessary repetition, the PCT/IB2016/053139 application isincorporated herein by reference in its entirety and reference will bemade throughout the present disclosure to the latter publication.

Thermal transfer printers are known that employ a ribbon carrying apolymeric ink film. The ribbon is equivalent to the ink ribbon used in aconventional typewriter, but the ink is solid ink and is transferredfrom it onto a substrate (usually paper) not by impact but by means of athermal print head that heats only the regions of the ribbon from whichthe ink is to be transferred to the paper. Thermal transfer printers canprint in monochrome or in full color, by transferring imagessuccessively from colored ribbons.

Such printers achieve printing of high quality but are wasteful, andtherefore costly to operate, since the ribbon is generally single-useand when discarded, much of its ink surface has not been transferred toa printing substrate.

WO2016/189512 discloses a printing system and method that operate on thesame principle as thermal transfer printers, but in which the single-useribbon is replaced by a transfer member which, rather than carrying apolymeric ink film, is coated with a layer of thermoplastic orthermoplastic-coated particles, which can be replenished after eachtransfer cycle, enabling the transfer member to perform multipleprinting cycles, significantly reducing waste.

In particular, WO2016/189512 discloses inter alia a method of thermaltransfer printing onto a surface of a substrate, which method comprisesthe steps of:

a) providing a transfer member having front and rear sides with animaging surface on the front side,

b) coating the imaging surface of the transfer member with individualparticles formed of, or coated with, a thermoplastic polymer,

c) removing substantially all particles that are not in direct contactwith the imaging surface to leave a uniform monolayer particle coatingon the imaging surface,

d) applying radiation to selected regions of the coated imaging surfaceto heat and render tacky the particles within the selected regions, and

e) pressing at least a portion of the coated imaging surface and atleast a corresponding portion of the substrate surface against oneanother, to cause transfer to the surface of the substrate of only theregions of the particle coating that have been rendered tacky.

To permit continuous printing, following transfer of particles from theselected regions to a first substrate surface, steps b) and c) may berepeated to apply a fresh monolayer coating of particles at least to theselected regions from which the previously applied monolayer coating wastransferred to the substrate surface in step e), so as to leave theimaging surface again uniformly coated with a monolayer of particles forprinting onto a subsequent substrate surface, as described in steps d)and e). In other words, for printing of subsequent images (which neednot be identical from cycle to cycle), steps b) to e) can besequentially repeated.

WO2016/189512 suggested pressing the coated imaging surface and thesubstrate surface against one another during application of radiation.This requires the radiation to the applied to the rear side of thetransfer member. In practice this mode of operation has been found to bethe more preferable but WO2016/189512 does not provide a clear teachingas to how efficient irradiation from the rear side of the transfermember can be achieved.

SUMMARY

In accordance with a first aspect of the present disclosure, there isprovided a printing system for thermal transfer printing onto a surfaceof a substrate, the system comprising:

a) a movable transfer member having opposite front and rear sides withan imaging surface on the front side,

b) a coating station adapted to apply a monolayer of particles made of,or coated with, a thermoplastic polymer to the imaging surface or atleast a segment thereof,

c) an imaging station adapted to apply energy in the form ofelectromagnetic (EM) radiation via the rear side of the transfer memberto selected regions of the particles coated imaging surface to renderthe particles thereon tacky within the selected regions, and

d) a transfer station adapted to press the imaging surface and thesurface of the substrate, or respective segments thereof, against oneanother to cause transfer to the surface of the substrate of only theregions of the particle coating that have been rendered tacky,

wherein

(e) the rear side of the transfer member is formed of a body transparentto the EM radiation, and

(f) an EM radiation absorbing layer made of an elastomeric silicone isprovided on the front side of the transfer member adjoining thetransparent body, the imaging surface being formed on, or as part of,the radiation absorbing layer.

In accordance with a second aspect of the present disclosure, there isprovided a printing system for thermal transfer printing onto a surfaceof a substrate, the system comprising:

a) a movable transfer member having opposite front and rear sides withan imaging surface on the front side,

b) a coating station adapted to apply a monolayer of particles made of,or coated with, a thermoplastic polymer to the imaging surface or atleast a segment thereof,

c) an imaging station adapted to apply energy in the form ofelectromagnetic (EM) radiation via the rear side of the transfer memberto selected regions of the particles coated imaging surface to renderthe particles thereon tacky within the selected regions, and

d) a transfer station adapted to press the imaging surface and thesurface of the substrate, or respective segments thereof, against oneanother to cause transfer to the surface of the substrate of only theregions of the particle coating that have been rendered tacky, whereinan EM radiation absorbing layer is provided on the front side of thetransfer member, the radiation absorbing layer adjoining a body,transparent to the EM radiation, on the rear side of the transfermember, the imaging surface being formed on, or as part of, theradiation absorbing layer; and wherein the transfer member satisfies atleast one, at least two or at least three or more of the followingstructural features:

-   -   i) the radiation absorbing layer is made of an elastomeric        silicone;    -   ii) the transparent body is made of an elastomeric silicone;    -   iii) the transfer member is substantially non-compressible;    -   iv) the radiation absorbing layer comprises sub-micron carbon        black particles dispersed in an elastomeric silicone;    -   v) the sub-micron carbon black particles have an average primary        particle size (Dv50) of at most 100 nm;    -   vi) the sub-micron carbon black particles have a predominant        cluster size (Dv90) of at most 500 nm;    -   vii) the radiation absorbing layer has an absorbance of at least        0.14 μm, the absorption being measured at a wavelength of said        EM radiation or within a proximal range thereof.

In different embodiments, the application of thermoplastic particles soas to form a monolayer of particles on an imaging surface of a transfermember of such printing system may comprise:

-   -   directing a fluid or fluid jet carrying the particles onto the        imaging surface,    -   rubbing the particles onto the imaging surface using a cloth,        brush or an application roller,    -   forming the imaging surface and the surface of the particles of        hydrophobic materials and directing a liquid containing the        particles onto the imaging surface, the liquid being selected so        as not to wet the imaging surface,    -   directing onto an intermediate applicator a gas or liquid jet        containing the particles, the applicator being capable of        receiving the particles and transferring them to the imaging        surface, or    -   any suitable combination of the above-mentioned techniques and        structures.

In accordance with a further aspect of the invention, there is provideda method of thermal transfer printing onto a surface of a substrate,which comprises:

a) providing a movable transfer member having opposite front and rearsides with an imaging surface on the front side,

b) applying to the imaging surface a monolayer coating of particles madeof, or coated with, a thermoplastic polymer,

c) applying EM radiation via the rear side of the transfer member toselected regions of the coated imaging surface to render the particlesthereon tacky within the selected regions, and

d) pressing the imaging surface and the surface of the substrate againstone another to cause transfer to the surface of the substrate of onlythe regions of the particle coating that have been rendered tacky,

wherein

(e) forming the rear side of the transfer member of a body transparentto the EM radiation, and

(f) providing a radiation absorbing layer made of an elastomericsilicone on the front side of the transfer member adjoining thetransparent body, the imaging surface being formed on, or as part of,the radiation absorbing layer such that the EM radiation reaches theimaging surface by passing through the transparent body.

The terms “tacky” and “sufficiently tacky” as used herein are notintended to mean that the particle coating is necessarily tacky to thetouch but only that it is softened sufficiently to enable its adhesionto the surface of a substrate when pressed against it in the transferstation. The tacky particles or regions of particles rendered tacky arebelieved to form individual films or contiguous films which followingtheir transfer to a printing substrate may optionally yield thinnerfilms, as a result of the pressure being applied upon contacting of theimaging surface (or a segment thereof) to the substrate (or acorresponding segment thereof) and/or of the optional further processing(e.g., fusing, drying, curing, coating, etc.) of the transferred films.Such optional further processing may include heating of thealready-transferred images and/or the receiving substrate by means whichdo not contact the transferred image or by means which contact thetransferred images, both of which means are well known in the art. Inthe case of non-contact heating, such as hot air, radiant heating, radiofrequency heating and the like, heating the transferred image mayenhance its adhesion to the substrate, its adhesivity towardssubsequently applied materials, its abrasion resistance, its chemicalresistance and the like. In the case of heating means which contact theimage, such as silicone-coated fuser rolls or belts, in addition to thebenefits of non-contact heating, the image film may also acquire highergloss and scratch resistance.

The intended meaning of the term “monolayer” and different ways in whicha monolayer can be achieved are disclosed in WO2016/189512 andWO2016/189513 which provide details of the particle size, polymer filmthickness as well as the design and construction of an imaging stationfor emitting laser radiation.

The thermoplastic particles may have a particle size of less than 40 μm,20 μm, 10 μm, or less than 5 μm, or less than 1 μm, or within the rangeof 100 nm to 4 μm, or 300 nm to 1 μm, or 500 nm to 1.5 μm.

Particular imaging devices that may serve in such imaging stations arefurther detailed in WO2016/189510 and WO2016/189511, all foregoingapplications to the same Applicant having published on 1 Dec. 2016.

Briefly, in order to facilitate replenishment of the particle coating onthe imaging surface after each transfer, particles that adhere to theimaging surface more strongly than they do to one another are utilized.This results in an applied layer that is substantially a monolayer ofindividual particles, with little, if any, overlap, the thickness of themonolayer being therefore commensurate (e.g., 1-3-times) with thethickness of the particles. Stated differently, the layer is only oneparticle thick over a major proportion of the area of the imagingsurface and most, if not all, of the particles have at least some directcontact with the imaging surface.

Advantageously, a monolayer of particles facilitates the targeteddelivery of radiation. This may ease the control of the imaging device,as the selectively irradiated particles reside on a single definedlayer, which may facilitate focusing the laser radiation to form upontransfer to a substrate a dot of approximately even thickness and/orrelatively defined contour.

Another advantage of having a monolayer is that it can provide for goodthermal coupling between the particles and the imaging surface on whichthe particles are coated.

To permit the printing on the substrate of patterns corresponding to theselected regions exposed to radiation, the affinity of the heated tackyparticles needs to be greater to the substrate than to the imagingsurface. Moreover, this relatively higher affinity of the tacky particleto the substrate in the selected regions shall also be greater than theaffinity of the bare substrate to the particles not rendered tacky. Inthe present context, a substrate is termed “bare” if lacking any desiredimage pattern to be printed by the present method or system. Though thebare substrate should for most purposes have substantially no affinityto the thermoplastic particles, to enable the selective affinity of thetacky ones, some residual affinity can be tolerated (e.g., if notvisually detectable) or even desired for particular printing effects.Undesired transfer of particles to areas of the bare substrate is alsotermed parasite or parasitic transfer.

The term “thermoplastic particles” is used to refer to all particles(colored or not) comprising a thermoplastic polymer, whether coating theparticle or forming substantially all of the particle, including anyintermediate range of presence of the polymer allowing the thermoplasticparticles to serve their intended purposes. In the latter cases, whereinthe thermoplastic polymer(s) can be homogeneously present in the entireparticle, not being particularly restricted to an external coating, theparticles may also be said to be made of a thermoplastic polymer.Colored thermoplastic particles may alone, but also in combination,provide a desirable printing effect (e.g., a colored pattern or a partthereof). Uncolored transparent thermoplastic particles not necessarilybut typically provide a desirable printing effect when combined withother materials capable of displaying a visually detectable effect. Forillustration, transparent particles may form an uncolored under-coatimage capable of further adhering to subsequently applied particles(e.g., made of plastics, metals, or any material providing a desirableprinting effect) or may form an uncolored over-coat capable of modifyingthe mechanical or esthetical properties of an image previously formed onthe substrate. A transferred image of thermoplastic particles (whetheror not colored) that may serve to later adhere different particlesthereto may be termed an “adhesive image”.

Such gradient of affinities between the particles (before and afterheating), the fluid carrying the native particles for coating orreplenishing of the transfer member, the imaging surface, the printingsubstrate, and any such element of the method, can be modulated byselection of suitable materials or characteristics, such as hardness,smoothness, hydrophobicity, hydrophilicity, charge, polarity and anysuch properties known to affect interaction between any two elements.

For assisting in the transfer of the tacky film of particles from theimaging surface to the substrate, the imaging surface may behydrophobic.

In some embodiments, the thermoplastic particles may themselves behydrophobic. In such case, the relative affinity between the particlesin their different states and the imaging surface can be based, at leastpartially, on hydrophobic-hydrophobic interactions. In some embodiments,attachment of the monolayer of particles to the imaging surface isassisted by the relative low hardness of the imaging surface as isfurther detailed below. A relatively soft imaging surface may assist informing an intimate contact with each individual particle, such intimatecontact manifesting itself in a relatively large contact area betweenthe imaging surface and the particle, in contrast to the discretecontact formed between the particle and a relatively hard surface. Suchintimate contact may thus further intensify effects of any short-rangeattraction forces between the imaging surface and the particles, suchas, e.g., hydrophobic-hydrophobic interactions or Van der Waals forces.

In some embodiments, the thermoplastic particles and/or the imagingsurface can alternatively or additionally achieve desired relativeaffinity one to another (and to any other fluid or surface suitable fora printing process according to present teachings) by way ofcharge-based interactions. For instance, positively charged particlesmay favor negatively charged surfaces. In such case, the relativeaffinity between the particles in their different states and the imagingsurface can be based on charge-charge interactions.

In some embodiments, the surface of the printing substrate can betreated to favor the transfer of the films of tacky particles. Treatmentcan be physical (e.g., by corona) or chemical (e.g., the substrateincluding a suitable external coat).

In accordance with a further aspect of the present invention, there isprovided a transfer member for use in a printing system of the presentinvention, having the form of an endless belt or a drum and comprising abody transparent to electromagnetic (EM) radiation lying within apredetermined range of frequencies disposed on a rear side of thetransfer member, a radiation absorbing elastomeric silicone layer opaqueto the EM radiation adjoining the transparent body and disposed on oradjacent a front side of the transfer member, and a hydrophobic releaselayer formed on the front side of the transfer member, the release layerbeing in thermal contact with, or formed as part of, the radiationabsorbing layer.

The transfer member support layer may be a drum or an endless belt, theimage surface disposed thereon being continuous (e.g., the belt being aseamless belt) and substantially uniform over its entire surface.

In some embodiments, the radiation absorbing layer may have anabsorbance per μm (as measured in the manner set out below) of at least0.1/μm, the absorption being measured at a wavelength of said EMradiation or within a proximal range thereof. In some embodiments, theabsorption may be at least 0.2/μm, or at least 0.3/μm.

Advantageously, the imaging surface is compatible with the radiantenergy intermittently applied by the imaging station to heat desiredselected areas. By compatible, it is meant for instance, that theimaging surface is relatively resilient and/or inert to the radiation atthe irradiated frequency/wavelength range, for example, the transfermember and the imaging surface maintain mechanical characteristics suchas strength and flexibility under such radiation. Also, the imagingsurface may be able to prevent or minimize heat loss to the transfermember and promoting effective heating to the particles. Additionally,or alternatively, the imaging surface may be able to conduct heat thatis generated by the radiation, such conduction being advantageouslyrestricted to the thin layer adjoining the imaging surface.

The carbon black (CB), in some embodiments, has an average primaryparticle size (Dv50) of at most 100 nm.

The carbon black particles are desirably dispersed within the siliconematrix in such a manner that a predominant measured particle clustersize (Dv90) approximates to one half of the wavelength of the appliedradiation. Thus, assuming that IR radiation having a wavelength in therange 700 nm to 1100 nm, is used in the imaging system, the predominantmeasured cluster size (Dv90) of the CB particles in the radiationabsorbing layer preferably does not exceed a value within the range of350 nm to 550 nm.

The concentration of the carbon black particles within the siliconematrix of the radiation absorbing layer, in some embodiments, is atleast 0.01%, by weight of the silicone matrix, and optionally of at most30 wt. %. In some embodiments, the weight per weight concentration ofthe carbon black particles within the radiation absorbing layer isbetween 5% and 30%, between 7.5% and 27.5%, between 10% and 25%, between12.5% and 25%, between 15% and 25%, or between 15% and 20%. By way ofexample, the concentration of carbon black particles within theradiation absorbing layer can be approximately 10%, 11%, 12%, 13%, 14%,15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, or 25% w/w.

In some embodiments, the silicone matrix of the imaging layer, includingof the radiation absorbing layer, is an addition-cure silicone matrix.

In some embodiments, the silicone matrix of the imaging layer, includingof the radiation absorbing layer, is a condensation-cure siliconematrix.

Addition cured silicone can be identified by the presence of platinum inthe cured matrix, platinum being used in the curing process.Condensation cured silicone, on the other hand, can be identified by thepresence of tin in the cured matrix.

In some embodiments, the carbon black particles used for the preparationof the radiation absorbing layer are of hydrophilic CB and have at leastone, at least two, or at least three of the following structuralproperties:

-   -   A) a volatile matter content of at least 1.5%, by weight of the        carbon black particles, or at least 2.5%, at least 3.5%, at        least 5%, at least 8%, at least 10%, at least 12%, at least 15%,        or at least 18%, and optionally, at most 50%, at most 35%, at        most 30%, at most 27%, at most 25%, or at most 22% (as can be        determined, in particular embodiments, by standard methods as        detailed in DIN 53552);    -   B) an oxygen content of at least 1.0%, at least 1.5%, at least        2%, at least 3%, at least 4%, at least 5%, at least 7%, at least        10%, at least 12%, or at least 15%, and optionally, at most 40%,        at most 30%, at most 25%, at most 22%, or at most 20%;    -   C) readily form a dispersion in distilled water, the water being        at a neutral pH and the carbon black particles making up 5 wt. %        of the dispersion;    -   D) an acid value in mmol/g, of at least 0.05, at least 0.06, at        least 0.075, at least 0.1, at least 0.125, at least 0.15, or at        least 0.175, optionally at most 0.5, at most 0.4, at most 0.3,        or at most 0.25, and further optionally, within a range of 0.05        to 0.35, 0.06 to 0.35, 0.08 to 0.35, 0.1 to 0.35, 0.05 to 0.3,        0.06 to 0.3, 0.08 to 0.3, 0.1 to 0.3, 0.05 to 0.25, 0.08 to        0.25, 0.1 to 0.25, 0.12 to 0.25, or 0.15 to 0.25;    -   E) a pH value of at most 5.0, at most 4.5, at most 4.0, at most        3.5, at most 3.0, or at most 2.7, (as can be determined, in        particular embodiments, by standard methods as detailed in DIN        ISO 787-9);    -   F) a surface Zeta potential of at most −15 mV, at most −20 mV,        at most −25 mV, at most −30 mV, at most −35 mV, or at most −40        mV, and optionally, within a range of −70 to −15 mV, −70 to −20        mV, −70 to −25 mV, −70 to −30 mV, −70 to −35 mV, −60 to −15 mV,        −60 to −20 mV, −60 to −25 mV, or −60 to −30 mV, the surface zeta        potential being measured at a pH of 12 and at a concentration of        0.1 wt % carbon black; and    -   G) an ID/IG ratio of 0.8 or more, 1.0 or more, 1.2 or more,        wherein ID and IG represent the peak intensity maxima of the        D-band and G-band of the carbon black, as measured by Raman        spectroscopy.

The oxygen content as provided in the specification is expressed inweight per weight of the carbon black particles, and can be converted toatomic percent by multiplying by a factor of 0.75.

In some embodiments, at least 80% of the carbon black particles, bynumber, are disposed at a normal distance of at least 0.01 μm, at least0.02 μm, at least 0.03 μm, at least 0.04 μm, at least 0.05 μm, at least0.1 μm, at least 0.2 μm, at least 0.3 μm, at least 0.5 μm, or at least1.0 μm, from the release surface.

In some embodiments, a conformable layer transparent to the EM radiationis disposed between the radiation absorbing layer and the support layerof the transfer member.

In some embodiments, the conformable layer has a hardness of up to 50Shore A.

In some embodiments, the conformable layer has a hardness within a rangeof 5 to 50, 10 to 30, 10 to 40, 10 to 50, 15 to 50, 20 to 40, or 20 to50 Shore A.

In some embodiments, the transfer member is adapted and dimensioned suchthat the transfer member has a compressibility of 100-500 μm, 100-400μm, 100-300 μm, 150-300 μm, or 150-250 μm in a direction normal to theimaging layer.

In some embodiments, the transfer member further comprises a transparentcompressible layer having a compressibility of 10-80% in a directionnormal to the large plane of the compressible layer. A transfer memberis said to be “substantially non-compressible” when it lacks such acompressible layer and/or it lacks the afore-said compressibility in adirection normal to the large plane of the imaging surface.

Other aspects and features of particular non-limiting embodiments of theinvention are set out in the detailed description and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The description, together with the figures, makes apparent to a personhaving ordinary skill in the pertinent art how the teachings of thedisclosure may be practiced, by way of non-limiting examples. Thefigures are for the purpose of illustrative discussion and no attempt ismade to show structural details of an embodiment in more detail than isnecessary for a fundamental and enabling understanding of thedisclosure. For the sake of clarity and simplicity, some objectsdepicted in the figures may not be drawn to scale.

In the Figures:

FIG. 1 depicts schematically a printing system as previously disclosedby the Applicant in WO2016/189512;

FIG. 2 is a schematic representation of a transfer member having asupport layer that is “transparent” to radiation of laser emittingelements;

FIG. 3a is a schematic representation of a digital printing systemaccording to one embodiment of the invention using a transfer member asshown in FIG. 2;

FIG. 3b shows to an enlarged scale the nip area illustrated as part ofthe system in panel FIG. 3 a;

FIGS. 4, 5 a and 5 b are schematic representations of alternativeembodiments of a digital printing system as exemplified in FIG. 3 a;

FIGS. 6a and 6b show the printing system of FIG. 3a as part of exemplaryprinting assemblies in which the substrate is subjected to furtherprocessing after passing through the transfer station;

FIG. 6c schematically depicts an alternative embodiment of a printingassembly in which images can be sequentially applied on a samesubstrate;

FIGS. 7a-7d show patterns of grooves that may be formed or present onsurfaces contacting a rear side of a transfer member, according tovarious embodiments;

FIG. 8 is a view generally similar to FIG. 3a but of an alternativeembodiment in which the imaging station is disposed within a druminstead of within the circumference of a continuous endless belt; and

FIG. 9 is a view of an alternative embodiment of a printing assembly inwhich images can be sequentially applied on a same substrate

DETAILED DESCRIPTION Overall Description of a Printing System

FIG. 1 shows a printing system as disclosed in WO2016/189512 of whichthe printing system of the present disclosure is a development. In FIG.1, a drum 10 serving as a transfer member has an outer surface 12 thatacts as an imaging surface. As the drum 10 rotates clockwise, asrepresented by an arrow, it cyclically passes beneath a coating station14 where it acquires a monolayer coating of fine particles. Afterexiting the coating station 14, the imaging surface 12 passes beneath animaging station 16 where radiation is applied by the imaging station 16to selected regions of the imaging surface 12 to heat and render tackythe particle coating on the selected regions of the imaging surface 12.In FIG. 1, the radiation is applied by exposing the selected regions ofthe front side of the imaging surface 12 to laser radiation. Bycontrast, in the present disclosure, as described in more detail below,radiant energy can be applied to the rear side of the transfer member(on the surface not coated by particles).

Next, the imaging surface 12 passes through a transfer station 18,having a nip where a substrate 20 is compressed between the drum 10 andan impression cylinder 22. While not shown in the figure, the impressioncylinder may include on its outer surface a compressible layer. Thepressure applied at the transfer station 18 causes the selected regionsof the coating on the imaging surface 12 that have been rendered tackyby exposure to laser radiation in the imaging station 16, to transferfrom the imaging surface 12 to the substrate 20. The regions on theimaging surface 12 corresponding to the selected tacky areas transferredto the substrate consequently become exposed, being depleted by thetransfer of particles. The imaging surface 12 can then complete itscycle, by returning to the coating station 14 where a fresh monolayerparticle coating is applied to the exposed regions from which thepreviously applied particles were transferred to the substrate 20 in thetransfer station 18. This step can be viewed as a replenishment of theparticle coating. As detailed below, the substrate, also termed printingsubstrate, may be made of various materials (e.g., paper, cardboard,plastics, fabrics etc.), some optionally existing in coated and uncoatedform depending on desired characteristics, and can be supplied to thetransfer station 18 in different forms (e.g., as sheets or continuouswebs).

The thermoplastic polymeric particles selectively heated for transfer tothe substrate are said to form a film, or as further detailedhereinafter a polymer film. As used herein, the term “film” indicatesthat each spot of particle(s) exposed on the imaging surface may form athin layer or coating of material, which may be flexible at least untiltransfer to the substrate at the transfer station. The term “film”should not be taken to mean that spots of adjacent particles that areheated at the imaging station are to transfer collectively as acontinuous coating. It is believed that a thin film formed on theimaging surface (i.e. by one or more adjacent particles sufficientlyexposed to a laser beam) may at most retain its thickness or become eventhinner upon transfer. Hence the printing system and method according tothe present teachings advantageously enable the printing on a substrateof a thin layer of particles that have been rendered tacky. In someembodiments, the printed film can have a thickness of 1 micrometer orless, or of no more than 800 nm, or of no more than 600 nm, or of nomore than 400 nm, or of no more than 200 nm, or even of no more than 100nm.

The Coating Station

The coating station 14 (or any coating station 14′ as may be found inadditional printing systems which may be combined to form printingassemblies) is similar to the coating station described in WO2016/189512and WO2016/189513 and will not therefore be described in detail herein.Essentially, the coating station comprises a plurality of spray heads1401 that are aligned with each other along the axis of the drum 10. Thesprays 1402 of the spray heads are confined within a bell housing 1403,of which the lower rim 1404 is shaped to conform closely to the imagingsurface leaving only a narrow gap between the bell housing 1403 and thedrum 10. The spray heads 1401 can be connected to a common supply rail1405 which supplies to the spray heads 1401 a pressurized fluid carrier(gaseous or liquid) having suspended within it the fine particles to beused in coating the imaging surface 12.

The fluid and the surplus particles from the spray heads 1401, which areconfined within a plenum 1406 formed by the inner space of the housing1403, are extracted through an outlet pipe 1407, which is connected to asuitable suction source represented by an arrow, and can be recycledback to the spray heads 1401. Though herein referred to as spray heads,any other type of nozzle or orifice along the common supply pipe orconduit allowing applying the fluid suspended particles are encompassed.

As an alternative to the above-described direct spraying of the fluidand suspended particles onto the imaging surface, the coating station,may, as shown in FIG. 2 of WO2016/189512 comprise a rotatable applicatoroperative to wipe the fluid and suspended particles onto the surface.The rotatable applicator can alternatively be a brush having fiber orfoam made bristles.

In some embodiments, there can be included on the entry side of thecoating system 14, and typically at an external upstream location asshown in FIG. 1, a cooler 1422 allowing lowering the temperature of theimaging surface 12 before the previously exposed regions of the particlelayer are replenished.

It is possible to provide both a cooler 1422 on the entry side of thecoating system 14 and a heater 1424 on the exit side. Additionally, thedrum 10 may be temperature controlled by suitable coolers/heatersinternal to the drum, such temperature controlling arrangements beingoperated, if present, in a manner allowing the outer surface of theimaging surface, or portions thereof, to be maintained at any desiredtemperature.

The Coating Particles

The shape and composition of the coating particles are fully describedin WO2016/189512. The invention described herein may employ particlesthat are pigmented, dyed or colorless. Briefly, for printing of highquality, it is desirable for the particles to be as fine as possible tominimize the interstices between particles of the applied monolayercoating, and to be preferably smaller than the required imageresolution. Being dependent upon the desired image resolution, for someapplications a particle size of up to 10 micrometer (μm) is deemedappropriate, in particular for pigmented thermoplastic particles.However, for improved image quality, it is preferred for the particlesize to be a few micrometers and more preferably less than about 1 μm.In some embodiments, suitable particles can have an average diameterbetween 100 nm and 4 μm, 300 nm and 1 μm, in particular between 500 nmand 1.5 μm. On account of the manner in which such particles areproduced, they are likely to be substantially spherical but that is notessential and they may be shaped as platelets.

In the case of colorless particles, such as those used to form aprotective or decorative over-coating, such as a varnish or lacquer, itmay be desirable to use particles as large as 5 micrometers, 10 μm, 20μm, 30 μm or even 40 μm in average diameter. While colorless particlesmay be the sole type desired for a particular printing system orprinting job, and may have any suitable dimensions, in some embodiments,to be further detailed in the following, the colorless particles areused as last to be applied on printing substrates to which coloredparticles were already transferred or as first to be applied on printingsubstrates to which colored particles are to be transferred. While forthe former case, the colored particles are thermoplastic so as to betransferable by a thermal printing system according to the presentteachings, such limitation no longer applies for colored particles to beadhered to an adhesive image formed by prior transfer of thermoplasticparticles. Thus, in the latter case, the colored particles may be madeof any material adapted to provide the desired printing effect, and canfor instance be made of plastics, metals, ceramics or glasses.Furthermore, the second particles can have any desirable shape, and berelatively spherical, bead-like, rod-like or flake like, the longestdimensions of said shapes possibly, but not necessarily, exceeding thelongest dimension of the rather globular first particles. Forillustration, the second particles can be metallic flakes (made ofmetals or alloys, and oxides thereof) and provide for a shinymirror-like, a glittering, a pearlescent and/or a nacreous appearance.

Typically sizes are provided as average of the population of particlesand can be determined by any technique known in the art, such asmicroscopy, Dynamic Light Scattering (DLS) or Light Scattering (LS), theformer being more suited for relatively smaller particles (e.g., of upto 6 μm) and the latter being more suited for relatively largerparticles (e.g., of up to 3.5 mm). The average diameter of a populationof particles can be assessed by Dv50 (maximum particle hydrodynamicdiameter below which 50% of the sample volume exists) and the size of apredominant portion of the population by Dv90. When the particles understudy are in a relatively viscous or solid media, such as in a curedlayer, such particle sizes can be assessed by microscopy, the skilledperson knowing which microscope and technique to employ depending on thedimensions/magnifications to be observed. When the particles under studyare in a relatively liquid media, such particle sizes can convenientlybe measured by DLS or LS.

In some embodiments, the polymer film resulting from the conversion ofthe monolayer of particles by exposure to radiation, whether providingby itself a desired (e.g., colored) printing effect or forming anadhesive image for particles to be later applied, has a thickness of 2μm or less, or of less than 1 μm, or even of less than 750 nm. In otherembodiments, the thickness of the polymer film is of 100 nm or more, orof more than 200 nm, or even of more than 300 nm. The thickness of thepolymer film may be in the range of 300 nm-1,000 nm, or of 500 nm-1,500nm, or of 600 nm-800 nm, or of 700 nm-1,000 nm.

In embodiments, wherein the thermoplastic particles are colorless, beingintended for instance for over-coating or under-coating, the particlesare typically larger than pigmented particles, and the film obtainedfollowing transfer may have accordingly a greater thickness. In suchembodiments, the thickness of the polymer film can be of up to 40 μm, orof no more than 30 μm, or of no more than 20 μm, or of no more than 10μm.

In some embodiments, the particles may be substantially hydrophobic.

The Particle Carrier

The particle carrier, that is to say the fluid within which the coatingparticles are suspended, may be either a liquid or a gas. If liquid, thecarrier is preferably water based and if gaseous the carrier ispreferably air. In the interest of economy, surplus particles extracted(e.g., sucked) from the interior of the plenum of a housing may berecycled to the supply and/or applicator device.

The Imaging Station

The imaging device depicted at imaging station 16 in FIG. 1 is alsofully described in WO2016/189512, particular embodiments being furtherexplained in WO2016/189510 and WO2016/189511, and need not be describedherein in detail. The imaging station (or imaging system) 16 is composedof a support 1601 carrying an array of chips 1602 each having anarrangement of individually controllable laser sources capable ofemitting laser beams. The chips 1602 are individually or collectivelyassociated with an array of corresponding lenses 1603 that focus thelaser beams on the imaging surface 12 or its vicinity (e.g., to heat upthe particles applied thereupon and/or to heat up an underlyingradiation absorbing layer).

FIG. 1 shows the imaging station 16 as facing the front side of thetransfer member 10. This requires the imaging station to be locatedupstream of the transfer station 18 but then it is important to ensurethat the irradiated tacky thermoplastic particles on the imaging surface12 do not lose their tackiness during transit between the imagingstation 16 and the transfer station 18. In some embodiments of thepresent disclosure, the imaging station is located facing the rear sideof the transfer member 10, thereby enabling irradiation and transfer tooccur substantially simultaneously. However certain embodiments of theinvention utilize an imaging station which irradiates the front side ofthe transfer member.

The Imaging Surface

The imaging surface 12 (and similarly the particle receiving surface712′ and 712″) in some embodiments is a hydrophobic surface, madetypically of an elastomer that can be tailored to have properties asherein disclosed, generally prepared from a silicone-based(release-prone) material. The silicone-based matrix may have anythickness and/or hardness suitable to bond the intended particles. Thesuitable hardness is to provide a strong bond to the particles when theyare applied to the imaging surface 12 in the coating station 14, thebond being stronger than the tendency of the particles to adhere to oneanother. It is believed that for relatively thin imaging surfaces (e.g.,100 μm or less), the silicone-based material may have a medium to lowhardness; whereas for relatively thick imaging surfaces (e.g., up toabout 1 mm), the silicone-based material may have a relatively highhardness. In some embodiments, a relatively high hardness between about60 Shore A and about 80 Shore A is suitable for the imaging surface. Inother embodiments, a medium-low hardness of less than 60, 50, 40, 30, 20or even 10 Shore A is satisfactory. In a particular embodiment, theimaging surface has a hardness of about 30-40 Shore A, a lower hardnessbelieved to be preferable for spherical particles. The hardness is of atleast 5 Shore A.

The hydrophobicity of the imaging surface enables the tacky film createdby exposing the particles to radiation to transfer cleanly to thesubstrate without splitting. A surface is said to be hydrophobic whenthe angle formed by the meniscus at the liquid/air/solid interface, alsotermed wetting angle or contact angle, exceeds 90°, the reference liquidbeing typically distilled water. Under such conditions, which areconventionally measured using a goniometer or a drop shape analyzer andcan be assessed at a given temperature and pressure of relevance to theoperational conditions of the coating process, the water tends to beadand does not wet, hence does not adhere, to the surface.

Advantageously, an imaging surface or particle receiving surfacesuitable for use with a printing system or various components of theprinting assembly herein disclosed can be flexible enough to be mountedon a drum, appropriately extendible or inextensible if to be mounted asa belt, have sufficient abrasion resistance and/or resilience, be inertto the particles and/or fluids being employed, and/or be resistant toany operating condition of relevance (e.g., irradiation, pressure, heat,tension, and the like).

The imaging surface can absorb EM radiation at the wavelength of thelaser emitting elements. For instance, if the radiation is emitted inany portion of the near infrared (NIR) range within about 800-2,000 nm,then the imaging surface should absorb over at least such portion of theNIR spectrum. In this way, the heating up of the imaging surface assistsin the softening of the particles disposed thereupon, sufficient heatingrendering the particles suitably tacky so as to transfer to a printingsubstrate.

Advantageously, the EM radiation absorbing material is such that it mayabsorb over a relatively wide range of laser wavelengths, compatiblewith different types of particles, each eventually having a differentsub-range, even minute ones, of laser absorbance. Carbon black (CB),which has a broad absorption and is a strong absorber in the NIR region,can be used to provide desired corresponding properties to the energyabsorbing layer of the imaging surface. Incorporation of carbon blackinto silicone-based layers may also contribute to the thermalconductivity of the imaging surface and allow it to be modulated, if andas desired. Silicone-based elastomers comprising CB particles andmethods of preparing the same are detailed in the following sections.

The imaging surface 12 in the drawing is the outer surface of a drum 10,which can be either directly cast thereupon or mounted as a separatelymanufactured sleeve. This, however, is not essential as it mayalternatively be the surface of an endless transfer member having theform of a belt guided over guide rollers and maintained under anappropriate tension at least while it passes through the coatingstation. Additional architectures may allow the imaging surface 12 andthe coating station 14 to be in relative movement one with the other.For instance, the imaging surface may form a movable platen which canrepeatedly pass beneath a static coating station, or form a staticplaten, the coating station repeatedly moving from one edge of theplaten to the other so as to entirely cover the imaging surface withparticles. Conceivably, both the imaging surface and the coating stationmay be moving with respect to one another and with respect to a staticpoint in space so as to reduce the time it may take to achieve entirecoating of the imaging surface with the particles dispensed by thecoating station. All such forms of imaging surfaces can be said to bemovable (e.g., rotatably, cyclically, endlessly, repeatedly movable orthe like) with respect to the coating station where any such imagingsurface can be coated with particles (or replenished with particles inexposed regions).

The transfer member, whether formed as a sleeve over a drum or a beltover guide rollers, may comprise in addition to the imaging surface, onthe side opposite the release layer, a body. As the transfer member inthe present disclosure is irradiated from its rear side (i.e. the sideopposite that carrying the monolayer of thermoplastic polymerparticles), the body needs to be transparent to the radiation, so thatthe radiation may reach the energy absorbing layer which is next to, orwhich incorporates, the imaging surface.

The body of the transfer member may comprise different layers eachproviding to the overall transfer member one or more desired propertiesselected, for instance, from mechanical resistance, thermalconductivity, compressibility (e.g., to improve “macroscopic” contactbetween the imaging surface and the impression cylinder), conformability(e.g., to improve “microscopic” contact between the imaging surface andthe printing substrate on the impression cylinder) and any suchcharacteristic readily understood by persons skilled in the art ofprinting transfer members.

The imaging surface may serve functions other than absorb energy andassist in the release of tacky particles. The imaging surface can, forinstance, be made of a material providing sufficient conformability,integrating the “conformable layer”, to its “release layer” and“radiation absorbing layer” functionalities. Conversely, the latter twofunctions may be provided by two distinct layers, the release layer(which will be in contact with the particles) and an underlyingradiation absorbing layer. Thus, an imaging surface can be asingle/unique layer encompassing at least both release and radiationderived functions, optionally supplemented by conformability duringimpression. Alternatively, the imaging layer may be formed from at leasttwo distinct layers selected from the group comprising release layers,radiation absorbing layers and conformable layers. In embodiments wherethe imaging surface consists of the three afore-mentioned types oflayers (named by their predominant function), then it may be preferredto have them ordered such that the release layer may contact theparticles, the radiation absorbing layer would be next (reducing thedistance with the particles on the imaging surface outer side) and theconformable layer would be last, this layer being typically attachableor attached to a support. The support, as mentioned, can be rigid (e.g.,the surface of a drum or any like mechanical part) or flexible (e.g.,the body of a belt).

The rear side of the transfer member in the present disclosure istransparent to the wavelengths of the source of EM radiation appliedthereto, whereas the radiation absorbing layer next to the imagingsurface is substantially opaque to such radiation. As laser beams havinga relatively wide range of emissions may be preferred, the oppositesides of the transfer member are advantageously “transparent” or“opaque” over a similar range. Assuming for instance, a laser emittingat a wavelength in the range of 800 nm to 2,000 nm, this radiationsource being positioned on the “rear side” of the transfer memberopposite to the imaging surface, a “transparent” member would allowsufficient progression of such beam from the rear side across memberthickness, or at least until such beam reaches the radiation absorbinglayer of the transfer member, over at least the same portion of therange.

Transparent Transfer Member

A transfer member having a transparent body is schematically illustratedin FIG. 2 by way of a cross-section through its layers. For convenience,a source of irradiation 640 and a single particle 650, which for clarityare not drawn to scale, are shown to illustrate how the transfer member700 can be used in a printing system. In FIG. 2, 702 represents arelease layer, which need not be transparent, but is capable oftransiently retaining the particles 650 until they are selectivelysoftened for release. 704 represents a radiation absorbing layer capableof harvesting the radiation to enable the softening of the particles,706 represents a transparent conformable layer capable of facilitatingcontact between the release layer and particles thereupon and thetopography of the surface of the printing substrate during transfer attransfer station 18. Though illustrated in FIG. 2 as distinct layers,the imaging surface 12 can be formed of a single/unique imaging layer720 integrating the functions of layers 702 and 704 or the functions oflayers 702, 704 and 706, the remaining layer 710 representing a supportlayer for all the afore-said layers which can jointly form a desiredtransparent transfer member 700.

The release layer 702 may have, in some embodiments, a thickness of 3 μmor less, of 2 μm or less, or between 0.5 μm and 1.5 μm.

A release layer 702 may be made of any material capable of providingsufficient adhesion to native (non-tacky) particles and enough releaseto particles softened by irradiation to selectively transfer. Highrelease elastomers provide a variety of suitable candidates, includingbut not limited to liquid silicone resins (LSR), room temperaturevulcanization (RTV) silicones, polydialkyl siloxanes (PDAS), includingfor instance polydimethyl siloxanes (PDMS) silicones, which can be, ifneeded, further functionalized by desired reactive groups (e.g., aminegroups, vinyl groups, silane or silanol groups, alkoxy groups, amidegroups, acrylate groups etc., and combinations thereof, as known in theart of silicones) to produce functionalized silicones. As used herein,the term “silicone” is used broadly to include such functionalizedsilicones, unless explicit or evident to the contrary. While generallyencompassed by the term “silicone”, such functionalized silicones mayalso be referred to as “silicone-based” polymers. Some functions can becross-linkable moieties, while others may provide different desiredproperties to the elastomer. Additionally, the function of the elastomeris non-reactive and can be based on atoms such as fluor. Theseelastomers can be classified into addition-curable silicones andcondensation-curable silicones, some chemical families enabling bothcuring methods. Advantageously, in some embodiments, a release layer canadditionally reduce or prevent parasitic transfer. The release layer 702is preferably devoid or substantially devoid of fillers.

Non-limiting examples of addition-curable silicone (ACS) include LSR andaddition-curable RTV, PDAS and PDMS silicones, whether or not furtherfunctionalized. ACS elastomers are cross-linked to form a matrix in thepresence of cross-linkers and any such agent (e.g., a platinum catalyst)promoting the bridging of the polymers, or on the contrary retarding it(e.g., for practical manufacturing purposes, by way of inhibition of thecuring facilitators), any and all such agents being termed herein“addition curing” agent(s). In one embodiment, the ACS is avinyl-functionalized silicone, which may be cured in the presence of atleast one addition-curing agent, under any curing conditions suitablefor said materials.

Non-limiting examples of condensation-curable silicones (CCS) includecondensation-curable RTV, PDAS and PDMS silicones, whether or notfurther functionalized. CCS elastomers can be cross-linked to form amatrix in the absence of additional cross-linkers, such effect beingprovided by suitable moieties or functional groups on the siliconebackbone. In some embodiments, condensation curing may further require acatalyst (e.g., a tin catalyst) and any such agent promoting thecondensation of suitable moieties of the polymers, any and all suchagents being termed herein “condensation curing” agent(s). In oneembodiment, the CCS is a silanol-functionalized silicone, in aparticular embodiment a silanol-terminated silicone. The silanolfunctionalized CCS may be cured in the presence of at least onecondensation-curing agent, under any curing conditions suitable for saidmaterials. In one embodiment, the CCS is a reactive amino-silicone.Addition curing agents and condensation curing agents respectivelysuitable for the curing of ACS and CCS elastomers are known and need notbe further detailed herein. Likewise curing conditions for suchmaterials are known to the skilled person and may, if needed, readily beoptimized for any particular use by routine experimentation.

Presence of catalysts can be detected by trace analysis of tin (for CCS)or platinum (for ACS) by known analytical methods, e.g., by InductivelyCoupled Plasma Spectroscopy (ICP).

A radiation absorbing layer 704 can have, in some embodiments, athickness of 25 μm or less, or between 200 nm and 1 μm, or between 500nm and 2 μm, or between 2 μm and 20 μm, or between 2 μm and 10 μm.

A radiation absorbing layer 704 can be made of any powder or elastomericmaterial capable of absorbing the radiation emitted by the laserelements of the imaging device, satisfactorily transferring heat and/orfor a sufficient duration to the imaging surface (illustrated in thefigure by the release layer 702) and the particles thereupon.Preferably, the materials forming such layer, and more generally thetransfer member, allow the heat generated by the application ofradiation by the imaging device to dissipate rapidly enough for theheating of the thermoplastic particles to be time and/or spot specific(e.g., enabling the formation of a desired pixel). Elastomers having ahigh absorbing ability (e.g., as assessed by the absorbance of thematerial per micron thickness) in the range of relevance, such as blacksilicone rubbers, are considered advantageous from a manufacturingstandpoint.

Absorbance Abs is herein defined as being equal to log₁₀(I_(in)/I_(out)) where I_(in) is the radiant flux received by thatmaterial and I_(out) is the radiant flux transmitted through thatmaterial. The absorbance is considered high if Abs is greater than0.1/μm, or greater than 0.2/μm or even greater than 0.3/μm. For example,total absorbance of, e.g., 90% of the radiation (Abs=1) may be obtainedby radiation absorbing layer 704 having Abs=0.1/μm and a thickness of 10μm, or Abs=0.2/μm and a thickness of 5 μm or Abs=0.3/μm and a thicknessof about 3.3 μm. A thinner radiation absorbing layer 704 may have anadvantage of contributing to a higher image resolution upon printing(compared to a thicker layer), because relatively little heat maydissipate sidewise within the layer while heating the thermoplasticparticles on the release layer 702. A thicker energy absorbing layer 704may have an advantage of storing a larger amount of heat (compared to athinner layer) thereby maintaining a required temperature for renderingthe ink particles tacky for a longer duration. Too high an absorbanceshould be avoided, as it may result in over-heating of the neighboringlayers and/or particles. In certain cases, such an over-heating maydamage the imaging surface, impairing the release layer, and reducingprint quality.

The same silicone resins as described for the release layer (e.g., ACSor CCS silicones, whether or not functionalized) may be used, thesesilicones being now supplemented with carbon black to act as an IRabsorbing material. Preferably, all layers are formed by the same curingmethod, otherwise an intermediate layer may be required to block themigration of curing agents of one type to the layer of the other type,as such diffusion may hamper subsequent curing of the second layer. Thephenomenon of poisoning is known and can be readily addressed by theskilled person.

The hardness of the imaging layer 720 of the transfer member 700, or ofthe layers forming the imaging surface 12, if separate, can berelatively low. A relatively soft imaging layer may assist in forming anintimate contact with the particles during operation of the printingsystem. In some embodiments, each of layers 702, 704, 706, and 720 mayhave a hardness of 50 Shore A or less, 40 Shore A or less, 30 Shore A orless and 20 Shore A or less and of at least 5 shore A.

The underlying layers 704, 706 and 710 of the transfer member 700 needto allow sufficient penetration of the relevant range of wavelengths to“activate” the radiation absorbing layer 704 from the rear side of thetransfer member, allowing enough heat to travel forward toward theimaging surface so as to soften the particles rendering themsufficiently tacky for transfer, when desired.

While a transparent transfer member can hypothetically include atransparent compressibility layer, materials known for their highcompressibility (e.g., having a relatively porous structure) aregenerally opaque and would hamper sufficient progression of radiationacross member thickness (hence operability of the imaging surface).Replacing such materials by transparent ones generally affects therelative compressibility of the layer, thus imposing thickercompressibility layers to obtain overall similar compressibility of thetransfer member. By way of example, while an opaque compressible layerhaving a thickness of 300 μm may be able to compress down to 100 μmunder the pressure conditions applicable at the transfer station 18, atransparent compressible layer, which may typically have 5%compressibility under the same conditions, would require a thickness of4 mm to enable its thickness to be similarly compressed by about 200 μm.

In the embodiment of FIG. 2, the compressibility function is “external”to the transfer member 700, such property being provided by the printingsystem relying on a compressible element 708, as illustrated in FIGS. 3aand 3 b.

In some embodiments, a transparent lubricant 730 (e.g., a polyether,such as polyethylene glycol or polypropylene glycol, or a silicone oil)can be used between the rear side of transfer member 700 (e.g., betweenthe rear side of support layer 710) and compressible element 708 (or anysurface that may contact the transfer member and be optionally furtherpressed thereto). For example, the compressible element 708 may have asurface intended for contacting, at least partially, the rear side oftransfer member 700. The surface may include one or two rounded edges(or in other words one or two curved or rounded corners) intended toform gap(s) with the rear side of transfer member 700. Such gap(s) mayfacilitate entry of the lubricant 730 in between the surface and therear side, and/or exit of lubricant 730 from in-between the contactingsurface and the rear side. One example of such surface of compressibleelement 708 intended for contacting, at least partially, the rear sideof transfer member 700 and having rounded edges is schematicallyillustrated in FIGS. 5a and 5b . In FIGS. 5a and 5b , the entire surfaceof compressible element 708 intended for at least partially contactingthe rear side of transfer member 700 is rounded. In other examples, themiddle of the surface intended for at least partially contacting therear side of transfer member 700 may be straight, and only one or bothof the edges of the surface may be rounded. Additionally, oralternatively, the surface of compressible element 708 intended to atleast partially contact the rear side of transfer member 700 may includeone or more grooves (also referred to herein as one or more striations)for directing the traversal of lubricant 730 between the surface and therear side, e.g., from one edge of the surface to the other edge. In someexamples, each of the one or more grooves is parallel to the directionof movement of the transfer member 700, whereas in other examples atleast one of the one or more grooves is not parallel to the direction ofmovement of the transfer member 700. Grooves parallel to the directionof movement are illustrated in FIG. 7a , wherein a plurality of grooves771 on a surface 770 a due to at least partially contact the rear sideof the transfer member are schematically depicted as being parallel tothe direction of movement of the transfer member shown by an arrow.Examples of grooves being not parallel to the direction of movement ofthe transfer member 700 are schematically depicted by striations 772,773 and 774, on at least partially contacting surfaces 770 b, 770 c and770 d, respectively shown in FIG. 7b , FIG. 7c and FIG. 7 d.

If the surface of the compressible element 708 includes a plurality ofgrooves, the grooves may, for instance, be parallel to one another,although not necessarily representative of the shortest distance betweenthe two edges. However, in other instances with a plurality of grooves,the grooves may not necessarily be parallel to one another. In anexample in which the compressible element 708 includes one or moregrooves, the depth or depths of the one or more grooves may be anysuitable depth or depths.

Usage of the lubricant 730 in between the rear side of the transfermember 700 and the compressible element 708 may enable planing of thecompressible element 708, or in other words may enable the compressibleelement 708 to skim over the rear side of the transfer member 700(similarly to the phenomenon in fluid bearings), thus reducing frictionbetween compressible element 708 and transfer member 700. In someexamples, the surface of compressible element 708 intended for at leastpartially contacting the transfer member 700 may include sharp edges(e.g., as shown in FIG. 2) rather than one or two rounded edges, and/orthe surface of compressible element 708 may not have grooves (e.g., thesurface may be flat). In such examples, the amount of lubricant 730which is able to enter in between the surface and the rear side of thetransfer member 700, traverse between the surface and the rear side ofthe transfer member 700, and/or exit from in between the surface and therear side of the transfer member 700 may be smaller, and the frictionmay be larger, compared to examples with one or more rounded edgesand/or grooves.

It is noted that friction between the transfer member 700 and thecompressible element 708 may reduce the product lifetime of the transfermember 700 and/or the compressible element 708. Therefore, it may beadvantageous to reduce friction through the usage of the lubricant 730,in order to allow for a longer product lifetime of the transfer member700 and/or the compressible element 708. Reduction of friction due tothe usage of the lubricant 730 may additionally or alternativelyadvantageously reduce the likelihood of stick-slip detrimentallyaffecting the contact between the transfer member 700 and thecompressible element 708.

The lubricant 730 may additionally or alternatively function as arefractive index matching element, thereby advantageously assisting inreducing the impact of any optical defects that may be created byabrasion.

Arrows 740 illustrate how pressure forces (e.g., as applied at transferstation 18 in a direction opposite to the arrows) may affect the shapeof the compressible element 708, as schematically shown by the dottedcontour. While not shown, the compressible element may have a rigidbacking to ensure that a substantially constant distance is kept betweenthe radiation source and the outer surface of the impression cylinderwhen engaged with the transfer member during impression.

A release layer 702 can have, in some embodiments, a thickness of nomore than 3 μm, generally between 1 μm and 2 μm. Release layer 702 canbe made of the ACS or CCS elastomers. In one embodiment, a release layer702 is made of cross-linkable PDAS and PDMS silicones, the siliconebackbone bearing any moiety suitable for the desired curing method. Insome embodiments, such silicones are fluorinated to any suitable extent.However, fluorinated silicones are less preferred since they displaycompatibility problems with silicone polymers of other chemicalfamilies. The release layer 702 is preferably devoid or substantiallydevoid of fillers that may negatively affect the activity of the CBparticles of the radiation absorbing layer 704.

A radiation absorbing layer 704 can have, in some embodiments, athickness of no more than 25 μm, generally of no more than 15 μm, andtypically within the range of 1 μm to 10 μm, or between 2 μm and 5 μm.

A radiation absorbing layer 704 can be made of the same ACS or CCSelastomers as the release layer and/or as the conformable layer, ifdistinct. In one embodiment, a radiation absorbing layer 704 is made ofcross-linkable PDAS and PDMS silicones, the silicone backbone bearingany moiety suitable for the desired curing method.

While the radiation absorbing material (such as CB) can be evenlydistributed along the layer cross-section, in some embodiments anon-uniform distribution may be preferred. Such a non-uniformdistribution may, for example, have a peak (representing a relativelyhigher density of particles) close to the imaging surface so that strongabsorption may occur close to the imaging surface.

A transparent conformable layer 706 can have, in some embodiments, athickness of no more than 150 μm, generally between 100 μm and 120 μm.

A transparent conformable layer 706 can be made of transparent ACS orCCS curable silicones or of polyurethanes. Materials suitable for thepreparation of transparent layers are preferably devoid or substantiallydevoid of fillers, as such particulate additives may reduce or preventthe absorption of the energy by the radiation absorbing layer at theoperating wavelengths of the imaging device/printing system. Thetransparent conformable layer should have a refractive index (RI)identical or similar (e.g., within ±5% or even ±0.5%) to the RI of thematrix of the radiation absorbing layer (without its CB contents).

In embodiments where the imaging surface 12 is in the form of asingle/unique imaging layer 720 combining 702 and 704, such imaginglayer 720 can have, in some embodiments, a thickness of no more than 15μm, generally between 1 μm and 10 μm, or between 2 μm and 5 μm. Such alayer would incorporate the materials suitable for its “constituent”layers in similar amounts or proportions, as described herein for someembodiments of the invention, materials blended for the sake of releasefunctionality will preferably be transparent. In embodiments where theimaging surface 12 further comprises layer 706 in the single/uniqueimaging layer 720, such imaging layer 720 can have, in some embodiments,a thickness of no more than 100 μm.

A transparent support layer 710 can have, in some embodiments, athickness between 400 μm and 600 μm, or 450 μm and 550 μm, or between480 μm and 520 μm.

A transparent support layer 710 can be made of PET, thermoplasticpolyurethanes (TPU), silicones or any other suitable material, suchmaterials being preferably devoid or substantially devoid of any fillerable to interfere with the desired operability of the radiationabsorbing layer.

A transparent transfer member 700 formed by combinations ofafore-described layers can have, in some embodiments, a thicknessbetween 500 μm and 2,000 μm, or between 500 μm and 1,500 μm, or between500 μm and 1,000 μm, or between 500 μm and 900 μm, or between 600 μm and800 nm.

Though a compressible element 708 can, in some embodiments, be externalto the transparent transfer member, the compressibility it shouldprovide when combined in operation with the transfer member 700 istypically of at least 50 μm, at least 100 μm, at least 150 μm, or atleast 200 μm. The compressibility, in some embodiments, need not exceed500 μm, and is generally no greater than 400 μm or 300 μm.

A compressible element 708 can be made of silicones or polyurethanes. Insome embodiments, such materials are selected to provide a similar RI asthe transfer member, even if physically separated therefrom, so as tomaintain a substantially uniform RI along the optical path travelled bythe laser beams.

Examples of Imaging Surfaces

The imaging surfaces prepared according to the above principles werehydrophobic surfaces made of an elastomer comprising silicone polymerscross-linked by condensation curing and by addition curing. Whencombining, in addition to release and conformational properties,radiation absorbing capabilities, the elastomeric composition formingthis outer surface included an absorbing material or absorbing fillerable to absorb radiation (e.g., radiation from laser beams) and totransfer heat generated thereby to the imaging surface with sufficientefficiency so as to soften the thermoplastic particles positionedthereupon to an extent they are rendered tacky enough to selectivelytransfer to a printing substrate. Exemplary compositions for an imagingsurface including such a radiation absorbing layer were formulated bydispersing carbon black (CB) particles in compatible silicone-basedpolymers as detailed herein-below.

As appreciated by a person skilled in the art of elastomer formulation,a “compatible” set of materials for any particular composition orformulation means that the presence of any such compatible compound doesnot negatively affect the efficacy of any other compound for any step ofpreparation or in the final composition. Compatibility can be chemical,physical or both. For instance, a dispersant suitable to disperse carbonblack into a curable silicone fluid would be compatible both with thecarbon black material and with the silicone polymers to be cured (aswell as with any other agent required to perfect such curing; allcollectively generally termed the “silicone media”). For instance, thedispersant would not be compatible if it is, among other things,preventing, reducing or retarding the curing of the silicone elastomer,not miscible with the elastomer (e.g., forming or being disposed in adistinct phase) or deleterious to the carbon black, and causing any likeundesired effects. In some embodiments, compatibility may additionallymean that the materials deemed compatible share a common property, suchas a common silicon-based chemistry or a similar physical parameter,such as a comparable RI.

A compatible dispersant (e.g., miscible in the silicone matrix, forminga single phase therewith) may have a branched chemical structure and atleast one carbon-black-affinic moiety having affinity to a hydrophilicsurface of the hydrophilic carbon black particles. A CB-affinic moietyis selected from an amino moiety, an acrylate moiety and an epoxymoiety. The hydrophilic surface of CB generally results fromoxygen-based functional groups, such as epoxy, hydroxy or carboxylicgroups. A branched silicone dispersant consists of a backbone and atleast one branching unit, wherein at least one of said backbone and saidone or more branching units is siloxane-based, or contains at least onesiloxane unit. Similarly, the at least one CB-affinic moiety can bedisposed within the backbone or within the branching unit(s). If theCB-affinic branching units are only positioned at terminal ends of thebackbone of the dispersant, the molecule might be considered linear.However, the terms “branched molecule” or “branched dispersant” are usedherein to encompass all types and/or localization of substitution asherein described. While generally, the siloxane-based chain and theCB-affinic moieties are each disposed on separate “mono-type” componentsof the branched molecule (e.g., the dispersant having a siloxane-basedbackbone and CB-affinic moieties on branching units, or vice versa:CB-affinic moieties disposed within the backbone and siloxane-containingbranching units) this “segregation” is not necessary. Suitable siliconedispersants may for example have disposed within their backbone bothsiloxane units and CB-affinic moieties, forming a “poly-type” backbone,the branching units stemming from any of the foregoing mono-type orpoly-type backbone being also possibly a combination ofsiloxane-containing branching units and CB affinic branching units.

While in the description provided below, several dispersing methods aredisclosed, these are not meant to be limiting. Suitable equipment mayinclude an ultrasonic disperser, a high shear homogenizer, a sonicator,a sand mill, an attritor media grinding mill, a pearl mill, a supermill, a ball mill, an impeller, a dispenser, a horizontal agitator KDmill, a colloid mill, a dynatron, a three-roll mill, an extruder and apress kneader, to name a few. The curable compositions that may beobtained by any suitable process, as exemplified below, can then bedeposited upon a substrate to form, following levelling and curing, thedesired layer.

Carbon Black

It is believed that a variety of CB materials may be suitable, amongother functions, as an absorbing material for an imaging surfaceaccording to the present teachings. The Applicant believes that thepresent teachings surprisingly enable the dispersion of hydrophilic CBparticles in hydrophobic elastomeric compositions. Hydrophilic CBs,which can readily disperse in water at concentrations of at least 5 wt.%, can be characterized by their oxygen content, resulting from theoxidizing treatment used for their manufacturing, which is deemed tocorrelate with the content of volatile compounds. By selecting oradjusting the content of oxygen atoms on the surface of the carbon atomsto an amount within a range of 1 to 40 atomic percent or 5 to 25 atomicpercent, and/or by selecting or adjusting the content of volatilecomponents in the carbon black to constitute from about 1.5% to 50%,1.5% to 40%, or 10% to 25%, by weight of the powder, the dispersibilityof the CB and/or the stability of the dispersion may be appreciablyimproved. A stably dispersed CB may facilitate the preparation of animaging surface or an absorbing layer so as to obtain a substantiallyuniform absorbing capacity over the entire surface thereof, even ifabsorbance may occur in fact underneath the outermost surface andnominal absorbance varies along the depth/thickness of the transfermember. An even behavior of the transfer member (e.g., to absorbradiation, to absorb thermal energy, to transfer heat, etc.) isdesirable to achieve quality printing.

The term “atomic %” for the surface oxygen relates to the ratio of thenumber of oxygen atoms (O) to the number of carbon atoms (C): (O/C)×100%existing on a surface of the carbon black particles (including at anydetectable depth in an interior portion of the particle). Generally,such values are provided by the CB manufacturers, but can beindependently determined by known methods such as X-ray photoelectronspectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR),organic elemental analysis, or electron spectroscopy for chemicalanalysis (ESCA).

A CB material can be treated to increase the atomic percentage of oxygenon its surface. Examples of suitable oxidizing agents, whether gaseousor liquid, include ozone, hydrogen peroxide, nitric acids, andhypochlorous acids. The carbon black can be oxidized, for instance, withozone or an ozone-containing gas at ambient temperature. There are alsomethods of wet oxidation in which the carbon black is exposed to ahypohalous acid salt, including, for instance, sodium hypochlorite andpotassium hypochlorite.

By way of example, a typical preparation involves mixing the carbonblack powder with hypohalous acids or salts thereof, preferably in anaqueous medium, and stirring the mixture for 1-24 hours (hrs) at atemperature of room temperature (circa 23° C.) to about 90° C., elevatedtemperatures of 50° C. or more being advantageous. The powder is thenseparated from the slurry, washed to remove unreacted oxidizing agentand allowed to dry. The degree of oxidation may be controlled byadjusting the concentration of the oxidizing agent, the ratio of thecarbon black particles to the oxidizing agent, the oxidationtemperature, the oxidation time, the stirring speed, and the like. Theamount of oxygen on the CB surface (whether oxidatively-treated or not)is preferably 5 atomic % or more, 7.5 atomic % or more, or 10 atomic %or more, from the viewpoint of dispersion suitability.

Examples of a carbon black having an amount of oxygen of less than 5atomic %, which may therefore benefit from being oxidatively-treated tobe rendered suitable, include carbon black manufactured by a knownmethod such as the contact method, furnace method, or thermal method.Specific examples of such low surface oxygen CB include Raven® 5750,Raven® 5250, Raven® 2000, Raven® 1500, Raven® 1250, Raven® 1200, Raven®1190 ULTRAII, Raven® 1170, Raven® 1255, Raven® 1080, Raven® 1060, andRaven® 700 (all manufactured by Columbian Chemicals Company), Regal®400R, Regal® 330R, Regal® 660R, Mogul® L, Black Pearls® L, Monarch® 700,Monarch® 800, Monarch® 880, Monarch® 900, Monarch® 1000, Monarch® 1100,Monarch® 1300, and Monarch® 1400 (all manufactured by CabotCorporation), Color Black FW1 (pH 3.5, BET surface area 320 m²/g), ColorBlack 18, Color Black S150, Color Black S160, Color Black S170, Printex®35, Printex® U, Printex® V, Printex® 140U, Printex® 140V, NIPex® 180-IQ,NIPex® 170-IQ (all manufactured by Evonik Degussa Corporation), No. 25,No. 33, No. 40, No. 45, No. 47, No. 52, No. 900, No. 2200B, No. 2300,No. 990, No. 980, No. 970, No. 960, No. 950, No. 850, MCF-88, MA600, MA7, MA 8, and MA 100 (all manufactured by Mitsubishi ChemicalCorporation).

Carbon black having an amount of surface oxygen of 5 atomic % or more,may be prepared by oxidative treatment as mentioned, or is acommercially available product. Specific examples thereof include ColorBlack FW2 (amount of volatile material 16.5 wt. %, OAN 155 cc/100 g, pH2.5, BET 350 m²/g, PPS 13 nm), Colour Black FW 182 (amount of surfaceoxygen: 12 atomic %, amount of volatile material 20 wt. %, OAN 142cc/100 g, pH 2.5, BET 550 m²/g, PPS 15 nm), Colour Black FW 200 (amountof surface oxygen: 12 atomic %, amount of volatile material 20 wt. %,OAN 160 cc/100 g, pH 2.5, BET 550 m²/g, PPS 13 nm), NIPex® 150 (amountof volatile material 10 wt. %, OAN 120 cc/100 g, pH 4.0, BET 175 m²/g,PPS 25 nm), Special Black 4 or 4A (amount of volatile material 14 wt. %,OAN 100-115 cc/100 g, pH 3.0, BET 180 m²/g, PPS 25 nm), Special Black 5(amount of volatile material 15 wt. %, OAN 130 cc/100 g, pH 2.5, BET 240m²/g, PPS 20 nm), Special Black 6 (amount of surface oxygen: 11 atomic%, amount of volatile material 18 wt. %, OAN 170 cc/100 g, pH 2.5, BET300 m²/g, PPS 17 nm), all foregoing available from Orion EngineeredCarbons Co., Ltd; Raven® 5000 Ultra II or Ultra III (amount of volatilematerial 10.5 wt. %, OAN 95 cc/100 g, pH 3.0-3.5, BET 583 m²/g, PPS 8nm; manufactured by Columbian Chemicals Company), and Fuji Jet Black(amount of surface oxygen: 12 atomic %; manufactured by Fuji PigmentCo., Ltd.). Information regarding different properties of theseexemplary Carbon Blacks was provided by their respective manufacturers.

Additional CB particles that may be used for the preparation ofradiation absorbing layers of transfer members configured for thepresent invention include Black Pearls® 800, Black Pearls® 880, BlackPearls' 2000, Black Pearls' 4350, Black Pearls® 4750, Monarch® 460,Monarch® 480, Monarch® 570, Monarch® 580, Elftex® 415, Elftex® 430,Elftex® 460, Elftex® 570, Elftex® OP, Elftex® Vulcan P, Regal® 99R andRegal® 500R (all manufactured by Cabot Corporation), Raven® 890, Raven®890H, Raven® 1000, Raven® 1020, Raven® 1035, Raven® 1040, Raven® 1255,Raven® 3500 and Raven® 7000 (all manufactured by Columbian ChemicalsCompany), NIPex® 160-IQ, NIPex® 35, NIPex® 70, NIPex® 90, Printex® 60-A,XPB 229 and XPB 255 (all manufactured by Orion Engineered Carbons Co.).

The level of oxidation of the CB material can be estimated by Ramanspectroscopy (e.g., using LabRAM HR Evolution, Horiba Scientific). Thistechnique allows determining the D-band and G-band peaks of the compoundunder study for predetermined excitation laser wavelengths (e.g., in therange of 488 nm to 647 nm), laser powers (e.g., 40 mW) and integrationtimes (e.g., of 10 s to 120 s). Temperature can be controlled to reduceblack noise (e.g., by cooling the detector). The Raman peak intensitymaxima (I) can be obtained, with or without deconvolution of thespectrum by an integrated software further allowing baseline correction,if needed. It is then possible to compute the Raman peak intensity ratioof the D-band and G-band, respectively ID and IG. The maximal intensityof each peak is typically measured on the undeconvoluted spectra. Thespectral behavior and resulting band ratio (ID/IG) can be empiricallycorrelated with the level of oxidation of the elemental carbonmaterials. A relatively low D-band to G-band ratio indicates that the CBis less oxidized than a CB having a relatively higher D-Band to G-Bandratio, all other structural properties of the CB being similar. By wayof example, an ID/IG ratio of 0.8 or more, 1.0 or more, 1.2 or more,indicates that the CB material is relatively oxidized as desired in someembodiments of the invention. Such Raman spectra can be unaffected inthe bands of interest by some elastomer matrices (notably PDMS), so thatthe method advantageously provides a non-destructive technique to assessCB characteristics within a cured composition. Such an analysis wasperformed on a sample of Colour Black FW 182 (having a volatile mattercontent of ˜20 wt. %) and the ID/IG ratio of the CB material was foundto be 0.99. For comparison, a less oxidized sample (Mogul® L having avolatile content of—−4.5%) displayed a lower ID/IG ratio of 0.75.

Another way of characterizing carbon black is by its surface zetapotential, which is the measure of the magnitude of the electrostatic orcharge repulsion/attraction between particles. Zeta potential valuesprovide insight into the CB's ability to disperse, aggregate orflocculate.

In some embodiments, the CB has a surface zeta potential of at most −15mV, at most −20 mV, or at most −25 mV, and more typically, of at most−30 mV, at most −35 mV, at most −40 mV, or at most −45 mV.

In some embodiments, the surface zeta potential of the CB is within arange of −70 mV to −15 mV, −70 mV to −20 mV, −70 mV to −25 mV, −70 mV to−30 mV, −70 mV to −35 mV, −70 mV to −40 mV, −70 MV to −45 mV, −60 mV to−20 mV, −60 mV to −30 mV, −60 mV to −35 mV, −55 mV to −30 mV, −50 mV to−25 mV, −50 mV to −30 mV, or −50 mV to −35 mV.

In some embodiments, the surface zeta potential can be measured at a pHof at least 8.0, said measurement being optionally performed at a pH of12.0. Conveniently, the measurement of the zeta potential of a materialor of a composition can be performed at low concentration of thematerial in an appropriate carrier or on a diluted form of thecomposition. For instance, a test sample may comprise 2 wt % or less ofsolid material or composition ingredients, 1 wt. % or less, or 0.1 wt %or less.

The content of the CB particles in the imaging surface mayadvantageously be sufficient to achieve the desired radiationabsorption, heat transfer, selective tackiness of the particles, whicheffects may in turn depend on a variety of operating conditions of aprinting system in which such transfer member would be used. Typically,the carbon black is present in the layer forming the imaging surface orin the radiation absorbing layer at a concentration between 0.5% and 20%by weight of the cured layer, or from 1 wt. % to 15 wt. %, or from 2 wt.% to 10 wt. %, or from 1 wt. % to 7.5 wt. %, or from 5 wt. % to 20 wt.%, or from 10 wt. % to 20 wt. %, or from 15 wt. % to 20 wt. %.

The pH of an aqueous dispersion of the CB, as determined at 25° C., canpreferably be in an acidic to around neutral range, for instance from pH2.0 to pH 8.5, from pH 2.5 to pH 7.5, and advantageously, in arelatively acidic range from pH 2.0 to pH 5.5, or from pH 2.0 to pH 4.5,or from pH 2.5 to pH 4.0, or from pH 2.0 to pH 3.5. The pH of a CBdispersion of pre-determined concentration can be measured with anysuitably calibrated pH-meter equipment, for instance, according to DINISO 787-9. Briefly, a 4 wt. % CB dispersion (in 1:1 distilledwater:methanol) can be stirred for 5 minutes with a magnetic stirrer atabout 600-1,000 rpm, whilst the pre-calibrated pH electrode is immersedin the tested dispersion. The reading of the pH value is taken oneminute after switching off the stirrer.

A dibutyl phthalate (DBP) absorption value of the CB material is notparticularly limited, but is typically from about 50 mL/100 g to about200 mL/100 g, or from 100 mL/100 g to 200 mL/100 g, or from 150 mL/100 gto 200 mL/100 g. Generally, such DBP values, or similar Oil AbsorptionNumbers (OAN), are provided by the CB manufacturers, but can beindependently determined by known methods such as according to JIS K6621A method or ASTM D 2414-65T.

Carbon black particles can be further characterized by specific surfacearea measurements, the most prevalent methods includingcetyltrimethylammonium bromide adsorption (CTAB), iodine adsorption andnitrogen adsorption. The CTAB method is described in ASTM D 3765. Theiodine method is described in ASTM D 1510, and results in the assignmentof an iodine number.

A specific surface area of the CB material is not particularly limited,but when determined by BET nitrogen absorption techniques, is preferablyfrom 50 m²/g to 650 m²/g, or from 100 m²/g to 550 m²/g. Generally, suchBET values are provided by the CB manufacturers, but can beindependently determined by known methods such as according to ASTMD3037.

The substantially even dispersion/uniform absorbing capability describedherein-above, can be facilitated by using CB in the formed layer havinga particle size of less than one micrometer. Such dimensions arepreferred not only with respect to primary particle size (PPS), but alsofor secondary particle size (SPS), which may result from agglomerationof such primary particles. Particles, both primary and secondary, havingfor a predominant portion of the population a particle size of less thanhalf the wavelength of the emitted beam are further preferred, asscattering is accordingly reduced. Hence, CB particles having a particlesize predominantly (e.g., as assessed by Dv90) of less than 500nanometers, less than 400 nm, less than 300 nm or less than 200 nm arefavored. CB particles having an average size (e.g., as assessed byDv50), typically a primary particle size (PPS), of 100 nm or less aredeemed in the nano-range, primary particles having an average size of 80nm or less, 60 nm or less, 40 nm or less, or 30 nm or less, beingparticularly preferred for close particle packing. Generally, the CBparticles have an average PPS of 5 nm or more, or 10 nm or more, or 15nm or more. The size of the particles, predominantly of the primaryparticles, may affect their ability to closely pack within theelastomer, relatively small particles being capable of higher packingdensity than their relatively larger counterparts. Advantageously, alower amount of relatively small particles may achieve a similar CBdensity as a higher amount of relatively large particles. Depending ontheir size, and additionally among other things on the viscosity of theelastomer, the conditions and duration of curing, the thickness of thelayer being cured and such manufacturing factors known to the skilledperson, the particles may segregate and form a gradient-likedistribution across the layer thickness. Larger CB secondary particlesmay tend to more rapidly migrate and accumulate towards the bottom ofthe layer, while relatively smaller particles may follow such a trend,if at all, at a slower pace, hence remaining in relatively higherconcentration in the upper section of the layer. In this context,“bottom” and “top” sections of the layer relate to their orientationduring curing, and not necessarily when installed and in operation in aprinting system. Such a segregation of the particles forming innerstrata of particle distribution along the depth of the imaging surfacemay be advantageous if a sufficient thickness of the upper sectionbecomes substantially devoid of CB particles. This “top stratum” canserve as a release layer, the absence of particles increasing itssmoothness. In some cases, a relatively high smoothness of the releasingsurface of the imaging layer can be desirable. Smooth surfaces generallydisplay an arithmetical mean deviation Ra of less than 1 micrometer. Insome embodiments, the surface roughness Ra of the imaging surface isless than 0.5 μm, or less than 0.2 μm, or less than 0.1 μm.

Manufacturers generally provide the average primary particle size of theCB material, as assessed for instance according to ASTM D 3849. Particlesize distribution, whether assessed by DLS or microscopic techniques,may provide information on the primary particle size (PPS) of thematerial and on its secondary particle size (SPS), i.e. the size ofassembly of primary particles forming for instance clusters oragglomerates.

The CB particles may have any suitable aspect ratio, i.e., adimensionless ratio between the smallest dimension of the particle andthe longest dimension in the largest plane orthogonal to the smallestdimension. In some embodiments, the carbon black primary particles areapproximately spherical and can have an aspect ratio in the range of0.2:1 to 1:5, or 0.5:1 to 1:2. Secondary particles of CB which mayagglomerate therefrom are not necessarily spherical, still their aspectratio can be in the range of 0.1:1 to 1:10, 0.2:1 to 1:5, or 0.5:1 to1:2.

Though not essential, the carbon black primary particles may preferablybe uniformly shaped and/or within a symmetrical distribution relative toa median value of the population. In some embodiments, the carbon blacksecondary particles are within a relatively narrow particle sizedistribution, such narrow PSD being advantageously maintained in thecured silicone elastomer.

First Exemplary Procedure Grinding Step

A silicone dispersant having good heat stability and compatibility withdimethyl silicone fluids was poured into a spinning tree-roll millgrinding machine (Model JRS230, manufactured by Changzhou LongxinMachinery Co. Ltd.), and operated for up to about one hour, at roomtemperature (circa 23° C.). The speed was adapted to the viscosity ofthe paste as the milling process proceeds, such that the speed wasdecreased from 800 rpm to 100 rpm as viscosity increased with theaddition of CB. One such dispersant was a functional pendantamine/dimethyl silicone copolymer having an amine number of 8 and akinematic viscosity at 25° C. of about 3700 mm²/s (GP-342, GeneseePolymers Corporation) which was added in an amount of 375 grams (g) soas to constitute 37.5% by weight of the total composition (wt. %).Carbon Black nano-powder (Colour Black FW 182, Orion Engineered Carbons,CAS No. 1333-86-4, 20 wt. % volatile matter, pH 2.5, 550 m²/g BETSurface, PPS 15 nm) was dried for at least 2 hrs at 120° C. 250 g of thedried CB powder were slowly added to the silicone fluid, such amount ofCB constituting 25 wt. % of the final composition. It is to be notedthat while the CB material is defined as being in the nano-range due toits primary particle size of about 15 nm, the powder initially mixedwith the dispersant mainly consisted of larger agglomerates, aggregatesor chunks of CB having size of above 5 μm or even greater than 10 μm, asestimated by microscope techniques. The CB-dispersant mixture was milleduntil the CB powder was sufficiently size-reduced to be homogeneouslydispersed in the silicone fluid and a black, high viscosity mass wasobtained. Such size reduction (as well as any other step of the process)was performed under a controlled temperature environment at atemperature suitable to the most heat-sensitive of the materialsemployed. In the present case, amino-silicones set such threshold ofheat-sensitivity, losing their activity at temperatures of about 70° C.or more. Hence the size-reduction step involving the amino-siliconedispersant was performed under controlled temperature of about 50° C.The CB primary particles formed agglomerates and the average size (e.g.,diameter) of such CB secondary particles following this step was ofabout 200-400 nanometers, as estimated by image analysis of the curedlayer later obtained under light microscope (Olympus® BX61 U-LH100-3).The light microscope analysis supported the even distribution of theclusters across the silicone matrix. Trained observers estimated thatsmaller clusters of 100-200 nm were also present in the matrix, thoughbelow formal level of detection. A top view picture was captured byscanning electron microscope (SEM; FEI Magellan™ 400 operated intunneling mode) and at least 10 particles deemed by a trained operatorto represent the majority of the CB population, such particles forming arepresentative set, were measured. The dimensions of isolated particlesforming the clusters were found to be in agreement with PPS as providedby the manufacturer, and the cluster sizes was as preliminarily assessedunder light microscope, confirming the presence of clusters as small as100 nm. Without wishing to be bound by any particular theory, it isbelieved that amine groups of the amino-silicone dispersant bind tocarboxy moieties of the carbon black, sufficiently enveloping the CBparticles so as to reduce or prevent their agglomeration. Carbon blackneed not necessarily be functionalized with organic carboxylic acid, asoxygen absorbed on its surface behaves in a similar manner.

A mixture of vinyl functional polydimethyl siloxane (Polymer XP RV 5000,Evonik® Hanse, CAS No. 68083-18-1) containing a small amount of the sameGP-342 dispersant (9:1 ratio by weight, respectively) was separatelyprepared with a high-shear homogenizer (T 50 digital Ultra-Turrax®equipped with R50 stirring shaft, IKA®-Werke GmbH) operated for abouttwenty minutes at a controlled temperature of 25° C. and at 10,000 rpm.It is believed that the presence of additional surfactant in the curablefluid prevents or reduces migration of this amine silicone polymer fromthe carbon particles to the vinyl functional PDMS, which diffusion, ifoverly extensive, could cause undesiredagglomeration/aggregation/flocculation of the carbon black particles.The mixture comprising the vinyl functional PDMS was added to the blackmass in an amount of about 375 g, so as to provide the remaining 37.5wt. % of the composition. The addition was performed in step-wisefashion under continuous milling at the same conditions, until the blackmass turned into a high-viscosity, shiny black paste (typically within 1hour) having a high concentration of carbon black.

Dilution Step

In order to increase the fluidity of the black paste (25 wt. % CB) andfacilitate spontaneous self-leveling after coating, the black siliconepaste prepared as above-detailed was diluted to a concentration of 5 wt.% CB or less. Dilution was performed with a “Silicone premix” which wasprepared as follows: a vinyl-terminated polydimethylsiloxane 5000 mm²/s(DMS V35, Gelest®, CAS No. 68083-19-2) in an amount of about 50 wt. %, avinyl functional polydimethyl siloxane containing both terminal andpendant vinyl groups (Polymer XP RV 5000, Evonik® Hanse, CAS No.68083-18-1) in an amount of about 21.4 wt. %, and a branched structurevinyl functional polydimethyl siloxane (VQM Resin-146, Gelest®, CAS No.68584-83-8) in an amount of about 28.6 wt. %, were mixed by thehigh-shear T 50 digital Ultra-Turrax® homogenizer operated at acontrolled temperature of 25° C. and at 10,000 rpm for about twentyminutes.

The concentrated black paste was mixed with the silicone premix toreduce the CB concentration to 5 wt. % CB, as follows: GP-342 was addedto the silicone premix so that their respective concentrations were 8wt. % and 72 wt. % of the final diluted composition. The concentratedblack paste was added so as to constitute 20 wt. % of the dilutedcomposition, all these additions being performed under continuousstirring with a high-shear homogenizer (T 50 digital Ultra-Turrax®—IKA)at a controlled temperature of 25° C. and at 10,000 rpm. The stirringwas maintained for approximately 2 hrs until the diluted black PDMSsilicone mixture was homogeneous (e.g., no black chunks or aggregateswere observed). Different final concentrations of carbon black weresimilarly prepared by accordingly adjusting the quantities of theafore-mentioned stock fluids or pastes.

Curing Step

A diluted black PDMS silicone mixture as above-prepared can be renderedcurable by the addition of: at least one catalyst, typically in anamount of about 0.0005 wt. % to 0.2 wt. %, or about 0.05 wt. % to about0.2 wt. % of the total curable composition, at least one retardant orcuring inhibitor to better control the curing conditions andprogression, typically in an amount of about 0.1 wt. % to 10 wt. %, orfrom about 1 wt. % to 10 wt. % and finally, at least one reactivecross-linker, typically in an amount of about 0.5 wt. % to 15 wt. %, orfrom about 5 wt. % to 15 wt. %, the addition of the reactivecross-linker initiating the addition curing of the black PDMS mixture.

The above-described 5 wt. % CB diluted black PDMS silicone mixture wasrendered curable by the addition of: a platinum catalyst, such as aplatinum divinyltetramethyl-disiloxane complex (SIP 6831.2, Gelest®, CASNo. 68478-92-2) in an amount of about 0.1 wt. %, a retardant, such asInhibitor 600 of Evonik® Hanse, in an amount of about 3.7 wt. %, andfinally, a reactive cross-linker, such as amethyl-hydrosiloxane-dimethylsiloxane copolymer (HMS 301, Gelest®, CASNo. 68037-59-2) in an amount of about 8.7 wt. % of the total curablecomposition.

This addition-curable composition was shortly thereafter applied uponthe desired transparent mechanical support with an automatic filmapplicator (Model: BGD281, Shanghai Jiuran Instrument Equipment Co.,Ltd.) operated at 5-100 mm/s draw-down speed, the layers so appliedforming predetermined thicknesses in the range of 5-200 micrometers.

As an example of a transparent body, a sheet of polyethyleneterephthalate (PET, 100 & 150 micrometer thickness from Jolybar Ltd.)was used, such support being optionally pre-treated (e.g., by corona orwith a priming substance) to further the adherence, to its support, ofthe material including the radiation absorbing layer. Corona treatment,when applied to the body, included an exposure of about 20 minutes toUV-irradiation (UltraViolet Ozone Cleaning System T10X10/OES/E, suppliedby UVOCS® Inc.). A priming substance, when used to pre-treat the body,can comprise 2.5 wt. % tetra n-propyl silicate (CAS No. 682-01-9,Colcoat Co.), 2.5 wt. % vinyltrimethoxysilane (such as Dynasylan® VTMO,Evonik®), 5 wt. % titanium diisoproposy (bis-2,4-pentanedionate) (suchas Tyzor AKT855, Gelese), 2.5 wt. % platinum-divinyl tetramethyl (CASNo. 68478-92-2, such as SIP 6831.2, Gelest®) all in pure methanol AR(CAS No. 67-56-1, Bio-Lab Ltd.). The priming substance can be applied bywiping the surface of the recipient layer/body with a clean laboratoryfabric soaked with the priming fluid.

Transparent supports can be made of any optically clear suitablematerial (e.g., silicones such as polysiloxanes, polyethylenes, such aspolyethylene terephthalate (PET) and polyethylene naphthalate (PEN),polyacrylates, such as poly(methylacrylate) (PMA) and poly(methylmethacrylate) (PMMA), polyurethanes (PU), polycarbonates (PC),polyvinyls, such as polyvinyl chloride (PVC), polyvinyl alcohol andpolyvinyl acetate, polyesters, polystyrenes includingacrylonitrile-butadiene-styrene copolymer, polyolefins (PO),fluoro-polymers, polyamides, polyimides, polysulfones or the like,copolymers thereof or blends thereof. A material is said to be opticallyclear if it allows light to pass through the material without beingscattered (ideally 100% transmission). While transparency is generallyassessed with respect to visible light, in the present context amaterial would be suitably transparent if having atransparency/transmission of at least 85%, at least 90% or at least 95%to the wavelengths of relevance to the emitting beams used in anyparticular system. Transparency can be assessed by measuring the opticaltransmittance of a predetermined thin sample of the material (e.g., aflat square having edges of 1 cm and a thickness of 0.2-2 mm, or more ifdesired for elements external to the transfer member) using aspectrophotometer, over the wavelength range of relevance. A refractiveindex (RI) of about 1.35 to 1.45 indicates an opticallyclear/transparent material. Each layer of a transparent transfer memberthrough which radiation should progress should have similar or same RIvalues and/or transparency properties, so as to constitute amulti-layered transfer member having preferably even suchcharacteristics across its thickness. Such properties are consideredsimilar if within ±5%, or within ±2%, or even within ±0.5%.

The refractive index (RI) of materials is generally provided by themanufacturers, but can be independently assessed by methods known to theskilled person. For fluid materials (e.g., uncured/pre-cured silicones)methods such as described in ASTM D1218 may be suitable, while solidmaterials can be tested according to ASTM D542.

As explained, when using a transparent transfer member and rear-sideirradiation, a compressible element external to the transfer member canbe used instead of an internal compressible layer. In such case, thecompressible element needs to be transparent at least to the sameextent. Transparent supports, layers thereof, or external elements,preferably have a yellowness index of 1 or less.

The black polydimethyl siloxanes mixture, whether applied on apre-treated body or on a non-pre-treated body, was cured for 2 hrs at70° C. in a ventilated oven (UT 12 P, Thermo Scientific Heraeus® Heatingand Drying Ovens), followed by one hour post-curing at 120-140° C. toachieve a full cure and stable bonding of the layer to the support.

It is to be noted that the suitability of an amino-silicone polymer(deemed relatively hydrophobic) to disperse CB in size-reduced form in asilicone matrix is unexpected, in particular when the CB material isrelatively hydrophilic. As a rule, dispersions of carbon blacknanoparticles in silicones are difficult to achieve even when theparticles and the silicone media have similar hydrophobicity. Suchparticles tend to agglomerate with one another, rather than remaininghomogeneously dispersed in their primary particle size or any relativelysmall secondary particle size that would have been achieved by thedispersing step. To resolve this issue, conventional manufacturingmethods aim to increase the relative polarity of the silicone media,using therefore condensation-curable silicone polymers and associatedreagents.

In contrast, in the present example according to embodiments of theinvention, such dispersion of CB particles was achieved while usingaddition curing of the PDMS silicones and counter-intuitively usingamino-silicones as a dispersant. The obtained environment, which isrelatively hydrophobic/non-polar, was expected to be “adverse” torelatively “size-stable” dispersions of CB. It should be additionallynoted that the use of amino silicones is deemed counterintuitive becausetheir amine moieties, when unbound and thus free to interact, are knownto prevent or otherwise deleteriously affect addition-curing of thesilicone matrix. Hence, the inventors have found a delicate balanceconcerning the amount of amino silicone present during the preparationof a CB-loaded silicone matrix. On the one hand, the amount should beenough to envelop the CB particles and prevent, reduce or delay theiragglomeration/aggregation; on the other hand, an excess amount should beavoided to prevent, reduce or delay any deleterious effect on additioncuring that such unbound amino silicones may have. A suitableconcentration of amino silicones may depend on the type of CB particlesand silicone media, as well as on the relative concentrations of thecarbon black and curable silicone. This concentration may be determinedby routine experimentation. In some embodiments, the weight-per-weightratio between the carbon black and its dispersant (e.g., amino silicone,silicone acrylate etc.) is from 0.3:1 to 1:1, from 0.4:1 to 2:1, from0.7:1 to 1.8:1, or from 0.9:1 to 1.6:1; and/or the carbon black todispersant w/w ratio is approximately 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1,0.8:1, 0.9:1, 1:1 or 1:1.5. Without wishing to be bound to anyparticular theory, the attachment of the dispersant to the carbon blackparticles is believed to be non-covalent. Generally, even if thedispersant were to covalently bind to the carbon black particles to someextent, via chemical bonds between the molecules, the dispersant-ladenparticles of carbon black dispersed in a matrix as herein disclosed arenot covalently bound to the silicone polymers or pre-polymers, inparticular when addition-curing is being used. In other words,dispersant-laden carbon black particles prepared and/or dispersedaccording to the present teachings may be non-covalently, physicallyentrapped within the network of the cured silicone matrix of theradiation absorbing layer. While the absence of covalent binding inducesrelative mechanical weakness, which worsens as the amount of carbonblack particles is increased, the Inventors unexpectedly found that themechanical integrity of a radiation absorbing imaging layer according tothe present teachings is surprisingly sufficiently high for a transfermember prepared therewith to retain structural integrity even at highconcentrations of carbon black particles.

Amino silicones having a relatively low number of amine moieties(correlating with a low amine number) may be advantageous in achievingthis balance. In some embodiments, the amino-silicone dispersant has anamine number within a range of 3 to 150, 4 to 130, 5 to 100, or 6 to 80.

While the amine number of amino-silicones is generally provided by themanufacturer of such materials, it can also be determined by routineanalysis using standard methods. By way of non-limiting example, theamine number of a molecule harboring amine moieties can be assessed bytitration of the amino-silicone with hydrochloric acid, the amine numbercorresponding to the milliliters of 0.1N HCl needed to neutralize 10 gof product.

Suitable amino-silicone dispersants can be selected from the groupcomprising aminoethyl-aminopropyl-methylsiloxane-dimethylsiloxanecopolymers (CAS No. 71750-79-3), such as commercially available asGP-342 by Genesee, having a silicone backbone and CB-affinic aminomoieties as branching units; LP X 21879 by BYK Additives & Instruments(showing an absorption band at 1446 cm⁻¹ in FTIR, correlating to aminogroups); Silamine® 2972 and Silamine® STD-100 (by Siltech Corporation),easy release silicones, such as Tego® RC-902, premium release silicones,such as Tego® RC-922, siloxane acrylates, such as Tegomer® V-Si 2854(all by Evonik®); aminopropyl terminated polydimethylsiloxane, such asDMS-A32, DMS-A35, and DMS-A32R (all by Gelest®); aminopropyldimethyl-polysiloxane, such as KF-8015 (by Shin-Etsu Chemical Co.);amine functional silicones, such as GP-4 and GP-581 (by Genesis PolymersCorporation), Mirasil® ADM 211 (by Elkem Silicones), Skycore® SR266 andSkycore® SR220 (by Skycent Chemicals); amino siloxanes, such asStruksilon F 571, Struksilon F 589 and Struktol VP 5421 (bySchill+Seilacher “Struktol”); and combinations thereof.

Mono-amines may be preferred, in particular when the amine moiety isterminally positioned. Without wishing to be bound by any particulartheory, it is believed that once attached to carbon black, a terminalmono-amine is hindered and thus unavailable to negatively affect curing.

The surprising efficacy of the amino-silicone was further corroboratedwith the preparation of a first comparative formulation similar to theabove, in which the amino-silicone was replaced by a dispersant of adifferent chemical family known for its expected suitability with CB: apolyglycerin-modified silicone KF-6106, supplied by Shin-Etsu ChemicalCo. This conventional surfactant failed to satisfactorily disperse theCB particles of the present formulation.

In a second comparative example, a commercially available concentratedCB paste (Akrosperse 20-MI-005, 50% wt CB, Akrochem Corporation) wasmixed with the same ACS PDMS (DMS V35) in respective amounts yielding a5 wt. % final CB concentration. The CB paste was used as supplied,without addition of any dispersant of any type. The mixture wasdispersed using the spinning tree-roll mill similarly operated.Following this control process, the CB displayed relatively largeaggregates (˜0.5-1.5 μm, as microscopically assessed), which were atleast two-fold larger than the secondary particles formed using thepresent formulation and method.

Following the same rationale concerning the prevention of carbon blackself-agglomeration/self-aggregation (through formation of a dispersant'senvelop), it was found that in addition to amine functions ofamino-silicones, acrylate functions of silicone acrylates can achievesimilar CB dispersion. Exemplary silicone acrylates were formulated in aPDMS matrix as above-detailed, with minor modifications, such as theamount of the carbon black being of only 3 wt. % instead of previouslydescribed 5 wt. %. KP-578 supplied by Shin-Etsu Chemical Co., Tego® RC711 (˜1% acrylate) and Tego® RC 902 (˜4% acrylate), supplied by Evonik®Industries, achieved satisfactory CB dispersion at the tested CBconcentration.

In the case of silicone acrylates, an acrylate content of at least 0.5wt. % in the silicone dispersant is believed to be satisfactory, highercontents of more than 5 wt. % being deemed preferable. It is believedthat higher amount of an active CB-affinic moiety of a dispersant on anygiven backbone may allow reducing the amount of dispersant necessary forthe dispersion of same amount of CB particles. The content of acrylatein the silicone dispersants is typically provided by their suppliers,but can be determined by standard measuring procedures.

Second Exemplary Procedure

While the afore-mentioned method of preparing a radiation absorbinglayer or an imaging layer including the same, such layers able to laterform an imaging surface, was substantially devoid of added volatileorganic solvents, the following alternative procedure makes use of suchliquids. Such solvents, when compatible with the intended siliconefluid, may facilitate some stages of the layer preparation orapplication to recipient layers or supports, a relatively highvolatility being advantageous in reducing or eliminating the presence ofthese solvents in a final transfer member. A solvent is deemedsufficiently volatile if capable of fully evaporating, or substantiallyso, during curing.

In the present example, 50 g of CB (Colour Black FW 182, OrionEngineered Carbons) having a Dv10 of about 2.9 μm, a Dv50 of about 4.5μm, and a Dv90 of about 6.1 μm, as measured by DLS (Malvern ZetasizerNano S) were mixed with 50 g of amino-silicone dispersant (BYK LP X21879, having an amine number of about 36, BYK Additives & Instruments)in 200 g of xylene AR (having a boiling point of about 138.4° C., CASNo. 1330-20-7, Bio-Lab Ltd.). As in previous example, the CB powder wasdried for at least 2 hrs at 120° C. before being mixed with the siliconedispersant. The dispersion was carried out in an attritor bead mill(Attritor HD-01, Union Process®) with stainless steel beads of about4.76 mm (SS 302 3/16 inch beads, Glen Mills Inc.) at 700 rpm until theCB particles reached an average SPS (e.g., as assessed by D50) of lessthan 100 nm, generally of about 70 nm, which typically required about1.5-2.5 hrs, depending on the batch size. The size reduction wasperformed under controlled temperature of 50° C. The size distributionwas then assessed by DLS (Malvern Zetasizer Nano S) on a samplecomprising about 0.1 wt. % of CB and the CB particles co-milled with thedispersant were found to be predominantly in the nano-range (having aDv10 of about 48 nm, a Dv50 of about 74 nm, and a Dv90 of about 139 nm).

The CB dispersion was added to a two-part LSR silicone fluid, therelative amount of the added dispersion depending on the desired finalamount of CB in the matrix. In the present set of experiments, the CBconcentrations per weight of the final matrix (i.e. excluding thevolatile solvent) were about 2.4 wt. %, 4.5 wt. %, 6.5 wt. %, 8.3 wt. %,11.5 wt. % and 14.3 wt. %. The according weight of CB dispersions (i.e.about 6 g, 12 g and so on) was added to 20 g of Silopren® LSR 2540 (PartA), gently hand mixed, then poured into 20 g of Silopren® LSR 2540 (PartB), by Momentive Performance Materials Inc. It is noted that adding theCB materials to a pre-mix of Part A and Part B of the LSR was also foundto be satisfactory. The resulting CB silicone fluid was further mixedfor about three minutes in a planetary centrifugal mixer (ThinkyARE-250, Thinky Corporation) operated at 2,000 rpm at ambienttemperature and allowed to defoam under sole same centrifugal conditionsfor another three minutes. A sample was cured at 140° C. for about 2hrs. The pattern of dispersion of the CB particles in the siliconematrix was assessed by light microscopy as previously detailed and foundstable over the curing period of the LSR components.

To facilitate the application of the afore-mentioned CB dispersed LSRsilicone fluid, the stock was diluted in excess volatile solvent, xylenein the present case, typically at a weight per weight ratio of at least1:4, for instance at 1:9 wt./wt. The CB particles in the dilutedsilicone matrix appeared to remain stably dispersed for a period of timecorresponding at least to duration of casting, as assessed by lightmicroscopy.

The diluted CB-LSR-xylene suspension was applied to a smooth releasablesupport (e.g., non-treated PET sheet) by spray coating using an airpressure brush. Alternative application methods are possible (e.g., rodcoating and the like). While partial curing of the silicone matrix mayproceed at relatively low temperature of 100-120° C. (taking at most 2hrs, but generally about 0.5-1 hr, depending on layer thickness), suchstep can be accelerated by raising the temperature (e.g., reducingcuring duration to about 20 minutes if cured at 140° C.). A clearsilicone layer (due to serve as a conformational layer) was then cast ontop of such a partially cured radiation absorbing layer/imaging layer.One such silicone overcoat was a two-component clear liquid silicone,QSil 213, commercially available from Quantum Silicones. The resultingPET-supported layers were further partially cured at about 100° C. forapproximately 1-2 hrs. The PET support was then peeled away and the twolayers inverted so as to have the CB-loaded radiation absorbing layerfacing up and the clear conformational layer facing down, the latterlayer being then attached to the desired support (e.g., a transparentsupport) by any suitable method. In some embodiments, the attachment ofsuch layers to the support contributed to the completion of the curingof the imaging surface.

This alternative procedure allows the preparation of a silicone matrixhaving a relatively high load of carbon black particles, such particleshaving the advantage, as in the previously described method, of being inthe sub-micron range and even predominantly in the nano-range.

Third Exemplary Procedure

While the afore-mentioned methods of preparing a radiation absorbinglayer, or an imaging layer including the same, were based onaddition-curing of cross-linkable addition curable silicones, thepresent procedure alternatively involves condensation-curing ofcross-linkable condensation-curable silicones.

In a first step, the CB material was dried (at least 2 hrs at 120° C.),then size reduced in the presence of a silicone dispersant. In thepresent example, 50 g of CB (Colour Black FW 182) were mixed with 50 gof amino-silicone dispersant (BYK LPX 21879) in 100 g ofhexamethyl-disiloxane (HMDSO; having a boiling point of about 101° C.,CAS No. 107-46-0, Sigma-Aldrich Co. Ltd.). HMDSO was used as a volatileliquid diluent, in a manner similar to xylene in previous example. Thedispersion was carried out for 4 hrs in an attritor bead mill withstainless steel beads of about 4.76 mm (as previously described) at 700rpm until the CB particles reached an average SPS (e.g., as assessed byD50) of about 90 nm, as assessed by DLS. The size reduction wasperformed under controlled temperature of 25° C.

The size distribution was then assessed by DLS (Malvern Zetasizer NanoS) on a sample comprising about 0.1 wt. % of CB and thesurfactant-dispersed CB particles were found to be in the sub-micron tonano-range (having a Dv10 of about 52 nm, a Dv50 of about 91 nm, and aDv90 of about 211 nm).

In a second step, the CB dispersion was added to a silanol-terminatedpolydimethyl-siloxane, the relative amounts of the added dispersiondepending on the desired final amount of CB in the matrix. In thepresent set of experiments, the CB concentrations per weight of thefinal matrix were about 5.5 wt. %, 12.5 wt. % and 21.4 wt. %. Theaccording weight of CB dispersions (i.e. 40 g, 80 g and 120 g) was addedto silanol-terminated PDMS (DMS S-27, 700-800 mm²/s, Gelest®) inrespective amounts of 160 g, 120 g and 80 gr. The resulting CB siliconefluid was mixed for about ninety minutes in the attritor under the sameconditions (700 rpm and 25° C.) resulting in a black mass ofcondensation-curable PDMS.

To 9 g of CB-dispersed in the curable silicone, were added 1 g ofcross-linker (ethylpolysilicate PSI023, Gelest® or ethylsilicate 48,Colcoat) and 0.05 g of tin catalyst (dioctyl tin bis(acetylacetonate)Tin Kat® 223, CAS No. 54068-28-9, TIB). The curable mixture was degassedand applied to a desired support. Prior to the application of thedegassed mixture, a transparent PET was pretreated with ozone and coatedwith a priming layer (SS4120, Momentive) to facilitate attachment. Thecondensation-curable silicone layer was applied by a rod wire atpredetermined thicknesses of up to about 40 μm (including layers of 5 μmand 20 μm) and allowed to partially cure at ambient conditions (circa23° C. and 30-60% RH) for about 12-24 hrs. The partly cured structurewas transferred to an oven for 2 hrs at 120-140° C. and about 30% RH,for curing finalization. The pattern of dispersion of the CB particlesin the condensation-cured silicone matrix was assessed by lightmicroscopy as previously detailed and found stable, the particles beingwell-dispersed and without particles flocking.

While silicones comprising CB are commercially available, attempts tosize reduce their CB contents to such desired particle size ranges haveso far proven difficult. In a comparative example, a commerciallyavailable concentrated CB paste wherein CB is pre-dispersed in asilicone fluid (Akrosperse 20-MI-005, 50% wt CB, Akrochem Corporation)was mixed with the same CCS PDMS (DMS S-27) in respective amountsyielding a 5 wt. % final CB concentration. The CB paste was used assupplied, without addition of any dispersant of any type. The mixturewas dispersed using the spinning tree-roll mill operated as described inthe first experimental procedure. Following this control process, the CBdisplayed relatively large aggregates (˜0.5-1.5 μm, as microscopicallyassessed), which were, as previously observed with the ACS control, atleast two-fold larger than the secondary particles formed using thepresent formulation and method.

Without wishing to be bound by any particular theory, it is believedthat the conventional formulations lack CB particles having suitableproperties, and/or appropriate amounts and/or suitable agents able toprevent the reagglomeration of primary particles that may be transientlyobtained during any such milling.

Optical Measurements

Some optical properties of the radiation absorbing layers or imagingsurfaces prepared by the above-described methods were assessed. Unlessotherwise stated, the sample of interest was cast on a smooth support,such as a glass slide, and leveled by rod coating to a known thicknessand cured (e.g., 1-2 hrs at 120-140° C.), the cured layer havinggenerally a thickness of at least 2 μm, as established by confocalmicroscopy.

The cured layer was gently separated from its casting support and placedin a film holder suitable for subsequent measurements. The opticalabsorbance of such samples was measured with a spectrophotometer over arange of at least 300 nm to 1200 nm (Cary 5000, UV-Vis-NIRspectrophotometer from Agilent Technologies). The drop in intensitybetween the two sides of the film was normalized to the thickness of thetested samples and the absorbance of such layers per micrometer ofthickness (Abs/μm) was calculated.

Representative results of normalized absorbance at selected wavelengths,for layers including CB particles dispersed with amino-siliconedispersants, are presented in the table provided below in which thevalues reported for the matrices loaded with carbon black relate to theeffect of the sole CB particles (the baseline values of the respectivematrices being subtracted).

TABLE 1 Abs/μm Abs/μm Abs/μm Abs/μm Abs/μm No. Sample @ 300 nm @ 500 nm@ 700 nm @ 900 nm @ 1100 nm 1 2.5 wt. % CB in PDMS 0.293 0.093 0.0690.056 0.048 2 5.0 wt. % CB in PDMS 0.479 0.188 0.138 0.109 0.091 3 7.5wt. % CB in PDMS 0.692 0.291 0.204 0.158 0.129 4 Control: 10 wt. % CCB0.290 0.102 0.090 0.087 0.085 in PDMS 5 Ref 0 wt. % CB in LSR 0.001030.00149 0.00137 0.00150 0.000135 6 2.4 wt. % CB in LSR 0.067 0.041 0.0290.021 0.018 7 4.5 wt. % CB in LSR 0.196 0.106 0.074 0.056 0.047 8 6.5wt. % CB in LSR 0.439 0.224 0.156 0.117 0.096 9 8.3 wt. % CB in LSR0.651 0.326 0.222 0.165 0.133 10 11.5 wt. % CB in LSR 0.681 0.379 0.2610.195 0.159 11 14.3 wt. % CB in LSR 0.733 0.413 0.285 0.214 0.172 12 5.5wt. % CB in LSR 0.549 0.226 0.167 0.135 0.114 13 12.5 wt. % CB in LSR0.577 0.271 0.214 0.172 0.147

As can be seen in the above table, CB particles dispersed according tothe various methods herein disclosed provided comparable absorbingproperties per micrometer depth of layer, such absorbance generallydecreasing as the wavelengths increased. In the above, the methods ofpreparation and resulting layers were exemplified with three types ofsilicone polymers, two types of curing method and two types ofamino-silicones, see items 1-3 for addition curing of ACS PDMS, items6-11 for addition curing of ACS LSR and items 12-13 for condensationcuring of CCS PDMS. These examples also represent different types ofinteractions between the silicone dispersants and the CB particles.Amino-silicone dispersants are expected to form acid-base relationshipor amine-epoxy interactions. Silicone acrylate dispersants are believedto form dipole:dipole interactions.

All items representing exemplary embodiments of silicone matrix embeddeddispersions of CB particles prepared according to the present teachings,formed clear samples (i.e., lacking haziness/turbidity), as assessed byvisual inspection. Such results support the compatibility of thesilicone dispersants with the curable silicone elastomers, includingtheir miscibility therein. Such compatibility can also be preliminarilyassessed in a screening method of such materials, performed in theabsence of carbon black particles.

For comparison, similar silicone matrices prepared in the absence of CBparticles according to the present teachings displayed an insignificantto null baseline normalized absorbance, of about 0.001 Abs/μm or less,over the same range of wavelengths, see item 5 for LSR matrix, the PDMSmatrices behaving similarly whether cured by addition-curing or bycondensation-curing. The impact of the CB nanoparticles dispersedaccording to present teachings can be seen from the positive correlationbetween the wt. concentration of CB in the silicone matrix and theabsorbing capacity of the layer over the tested range. Based on thepresent set of results peak or plateau of absorbance for each particularformulation are expected at carbon loading of at least 10 wt. %, atleast 15 wt. % or possibly at carbon loading of more than 20 wt. %. SuchCB concentration dependent patterns can readily be established by theskilled person, who can elect desired CB loading as per peak of optimalactivity and/or intended use. For all practical purposes, it is believedthat carbon black presence in curable or cured silicone compositionsneed not exceed 30 wt. %, being often of no more than 25 wt. %.

Reverting to the table, in a control experiment, see item 4, acomparative layer was prepared in which the same carbon black materialwas milled and incorporated in a PDMS matrix similarly to items 1-3, themethod however lacking any amino-silicone dispersant. In the resultinglayer, the CB particles were therefore of a more conventional size, inthe range of 0.5-1.5 μm. This conventional CB (CCB) material wasembedded in the PDMS matrix at a relatively high concentration of 10 wt.%. Despite such high load, the CCB control provided a poorer absorptionrelatively to lower concentrations of CB particles prepared according tosome embodiments of the invention. In this experiment, the 10 wt. % CCBin PDMS was found comparable to the 2.5 wt. % CB in PDMS, see items 4and 1, respectively. Therefore, the present methods and formulations areapproximately 4-fold superior, with respect to the amount of CBparticles providing similar absorbance. The ability to reduce the amountof CB to achieve a particular radiation absorbance can have numerousbeneficial implications, beyond cost reduction, as readily appreciatedby the skilled person.

An adhesive layer can be used to attach the layers of the transfermember. Such layers have a thickness which may depend on the roughnessof the recipient layer, for relatively smooth recipient body, theadhesive layer can have a thickness typically not exceeding 10 μm. Anysuitable adhesive can be used, its composition being compatible with thelayers to be attached thereby. Furthermore, the adhesive layer, as anyother layers of the transfer member, is preferably adapted to theworking conditions to which the transfer member is subjected inoperation of the printing system.

An adhesive layer can be made of silicones, polyurethanes, and suchknown flexible elastomeric adhesive materials. Such examples are notlimiting, materials suitable to adhere elastomers one to another beingknown and in no need of being further detailed herein.

Alternatively, a priming layer can be used, the composition of whichdepends on the layers to be bound. Such layers typically have athickness of 1 μm or less. Suitable materials include silanes, titanatesand other such sizing agents.

In some embodiments, adhesive layers or priming layers are notnecessary, the attachment of one layer to another being achieved byco-curing of the two layers, at least one of which would have beenpreviously partially cured.

In embodiments of the present invention, the imaging and transferstations are combined and the imaging surface 12 (and the particlesthereon) is selectively heated substantially at the same time as it ispressed against the substrate for transfer of the films of tackyparticles from the selected regions of the imaging surface. This may beachieved, for example, by forming the drum 10 of a transparent materialand locating the imaging station 16 within the drum, as schematicallyillustrated in FIG. 8, or externally to the drum and across it at aposition “facing” the transfer station. By “transparent” it is meantthat the material of the drum and/or of the imaging surface does notsignificantly affect the irradiation of the selected particles and/orallow the transfer of sufficient power to render them tacky.

Some embodiments of such transparent alternative printing systems areschematically illustrated in FIGS. 3 to 5, to be described in moredetails in the following.

As described above, the thermoplastic polymer particles are renderedtacky by application of radiation to the rear side of the transfermember, the latter needs to be transparent, as is the case for thetransfer member 700 illustrated in FIG. 2. As shown in FIG. 3a , acoating station 14 can coat the imaging surface of a transparenttransfer member 700—being illustrated as a looped endless belt—withthermoplastic particles. The transfer member can continuously (orintermittently) cyclically circulate over a driving drum 30, serving atthe coating station a purpose similar to previously described drum 10,and over guide rollers 40. An imaging station 16 positioned within theperimeter formed by the continuous belt is schematically shown. Thelaser beams emitted by the imaging device at such a station areprojected towards the rear side of a run of the transfer member passingalong the gap formed between guide rollers 40. The imaging devicecomprises a stationary transparent compressible element 708, which cancontact the rear side of the transfer member at the transfer station.When referring to the stationary compressible element, the term“compressible” is used to describe the deformation which the elementundergoes when subjected to pressure, in which the dimension normal tothe process direction diminishes in response to increased transferpressure, while its orthogonal dimension increases. Thus, though theelement 708 is not compressed, in the sense of its density beingincreased, its thickness dimension is reduced.

While in the present illustration, two guide rollers 40 bound the run oftransfer member subjected to the imaging device or station 16 andcontacting its compressible element 708, this should not be construed aslimiting, as one or more guide rollers or smooth sliders may be used forthis effect.

FIG. 3b schematically shows a view of this printing system at theimpression nip 18, to an enlarged scale. At the nip 18, the transfermember 700 and the printing substrate 20 are compressed between animpression cylinder 22 on one side and the compressible element 708 onthe other side. As can be seen, the compressible element 708 of theimaging device may contact a multi-layered transfer member, thethermoplastic particles (not shown) being positioned on the outerimaging surface (that facing the printing substrate). As previouslymentioned, a transparent lubricant 730 can be used to facilitate thesliding of the transfer member rear side over the compressible element708, and/or compressible element 708 can include one or more groovesand/or rounded edges, to facilitate entry in between, traversal between,and/or exit from in between compressible element 708 and the rear sideof transfer member 700. The thickness of a compressible element 708 andof the layers forming a transparent transfer member to be used therewithare selected so as allow the radiation emitted by any laser element of achip of an imaging device to target the radiation absorbing layer 704,or any sufficiently adjacent strata, of the member to permit sufficientradiation absorbance, and subsequent heat delivery to the thermoplasticparticles.

In FIG. 4, an alternative configuration of rear side irradiation ofparticles positioned on an imaging surface through a transparenttransfer member, is schematically illustrated. The transparent transfermember 700 can be as described above by reference to FIG. 2. In thisembodiment, the compressible element previously shown as 708 in FIG. 3is no longer associated with the imaging device or station 16′, but witha separate pressure applicator 90, the compressible segment of which,identified as 92, serves a similar purpose as previous 708. Similarconcerns may therefore apply. For instance, a lubricant can be used tofacilitate the sliding of the transfer member rear side over thecompressible segment 92 of the pressure applicator 90, and/or, forinstance, compressible segment 92 may include one or more grooves and/orrounded edges, to facilitate entry in between, traversal between, and/orexit from in between compressible segment 92 and the rear side oftransfer member 700, as discussed above with reference to compressibleelement 708 and FIGS. 7a-7d . However, the compressible segment 92 maynow be relieved from certain constraints of previous 708. By way ofexample, the compressible material forming such segment need notnecessarily be transparent, permitting the use a wider range ofelastomers or other compressible materials and/or arrangements (such assupporting the compressible element by springs or gas by way ofexample). Regarding the imaging device 16′, the segment 94 no longerneeds to be as compressible, but mainly transparent to enable sufficientprogression of the laser beams towards the imaging surface. Segment 94may or may not be intended for at least partial contacting of thetransfer member rear side. If segment 94 is intended to at leastpartially contact the transfer member rear side, a lubricant can beused, for instance to facilitate the sliding of the transfer member rearside over segment 94 and/or, segment 94 may include, for instance, oneor more grooves and/or rounded edges, to facilitate entry in between,traversal between, and/or exit from in between segment 94 and the rearside of transfer member 700, as discussed above with reference tocompressible element 708 and FIGS. 7a-7d . Regardless of whether or notcontact is to be made, segment 94 can be made of a variety of materials,including, for example, glass and transparent plastics, such as acryl.Such materials are typically preferable, as far as choice and opticalimaging quality are concerned, over compressible transparent elastomersfrom which previous compressible element 708 would be formed. In theembodiment illustrated in FIG. 4, irradiation takes place immediatelyupstream of the impression nip.

While this embodiment is less compact than the alternative embodimentschematically illustrated in FIG. 3a , it offers a substantiallyconstant optical path length between the laser beam emitting element ofthe imaging device and the absorbing layer being targeted within thetransfer member. A printing system operating with such essentiallyinvariable optical path length is expected to benefit from a moreuniform spot aspect and a more even optical magnification, resulting onthe surface of the substrate in images of higher quality.

FIGS. 5a and 5b schematically depict separate embodiments of lubricationsystems that may be used to apply lubricant 730 to the rear surface oftransparent transfer member 700. Any one of the embodiments of FIGS. 5aand 5b may be used with the systems described in FIG. 3a and in FIG. 4.

In FIG. 5a , lubrication is applied to the rear surface of the transfermember 700 by a lubrication roller 800 positioned upstream, andpreferably close to the compressible element 708. Lubrication roller 800extends parallel to the rotational axes of the guide rollers 40 andacross the entire width of transfer member 700. Lubrication roller 800may comprise a hollow tube 802 in fluid communication with a lubricantreservoir (not shown here), and having a multitude of apertures alongits cylindrical surface. Hollow tube 802 may be further enveloped alongits cylindrical surface with a compressible sleeve 804 made of a porousmaterial, such a sponge. The sleeve 804 is thereby configured to allowliquid to drip in a generally radial direction from the hollow tube 802through the apertures and the sleeve 804 onto the rear side of thetransfer member 700.

Lubrication roller 800 is positioned so that compressible sleeve 804contacts the transfer member, and is configured to revolve about itsaxis. It may revolve correspondingly to the movement of the transfermember (e.g., by being driven by friction with the transfer member) orit may revolve independently of the movement of the transfer member sothat the surface of sleeve slides over the rear surface of the transfermember. In some embodiments, the lubrication roller 800 may revolve inthe opposite direction to the direction determined by the movement ofthe transfer member.

In operation, the hollow tube 802 may be substantially filled withlubricant 730 and lubricant 730 may correspondingly drip through theapertures of the hollow tube 802 and through compressible sleeve 804 tobe smeared on the rear surface of the transfer member. According to someembodiments lubricant 730 may be pressurized through the apertures,e.g., by a pump, and according to some embodiments the transparentlubricant drips through the apertures through gravitational force.

In FIG. 5b there are no guide rollers, and the transfer member slidesinstead over rounded corners 810 a and 810 b of a construction 812 thatlie upstream and downstream of the compressible element 708,respectively. Construction 812 may be employed in some embodiments tosecure the imaging system 16 to the compressible element 708. A hollowpassage 820 in construction 812 may be in fluid communication with alubricant reservoir (not shown here) and a lubricant 730 may escapethrough multiple of apertures in the wall of the passage 820 onto theexternal surface of the construction 812 over which the transfer member700 slides. Hollow passage 820 is advantageously positioned upstream ofthe compressible element 708 and even upstream of rounded corner 810 a,so that in operation lubricant 730 is applied to the transfer memberjust upstream to the point where the transfer member slides over therounded corner 810 a.

Lubricant 730 is configured to lubricate and facilitate the sliding ofthe transfer member over the compressible element 708 withoutinterfering, or at least with minimal interference with the optical pathof radiation from imaging system 16 towards the impression cylinder 22through the compressible element 708. While the lubrication systems havebeen illustrated in a system wherein the rear side of the transfermember is contacted by a compressible element 708, the same principlesapply to printing systems alternatively using a separate pressureapplicator 90, the rear side of the transfer member being then incontact with segments 92 and optionally 94.

Regardless of the specific architecture of the printing system in theregion spanning from the imaging system 16 to the rear side (e.g., asupport layer 710) of the transfer member 700, the lubricant 730 may, insome embodiments, be a liquid having a relatively low viscosity tofurther reduce friction between the transfer member 700 and any elementcontacting its rear side. A lubricant having a relatively low viscositymay have a dynamic viscosity of 400 mPA·s or less, 300 mPA·s or less,200 mPA·s or less, or 150 mPA·s or less. Usually, low viscositylubricants have a dynamic viscosity of 1 mPA·s or more, 10 mPA·s ormore, 20 mPA·s or more, 30 mPA·s or more, or 50 mPA·s or more. In someembodiments the viscosity of the lubricant lies within the range of30-400 mPa·S. More preferably, the viscosity may lie within the range of50-300 mPa·S. While such values are of greater relevance to theoperating temperature of the printing system, and more specifically tothe temperature the lubricant between the transfer member 700 and theelement(s) (e.g., 708; or 92 and/or 94) contacting its rear side, asuitable lubricant can be selected when such viscosity values aremeasured at about room temperature (circa 23° C.) with an appropriateviscometer. The shear rate experiences by the lubricant during operationof the printing system, inter alia as a result of the velocity of thetransfer member, may also affect the range of suitable viscosities, asreadily appreciated by a skilled person.

It is to be noted that the viscosity alone may not suffice to elect aparticular lubricant, as the relative polarity of the surfaces to belubricated and of the lubricant may also contribute to a desired levelof lubrication (in other words, reducing friction between the slidingand static parts). For instance, comparing lubricants having similarviscosities, semi-polar ones (e.g., silicone polyethers, such asSilsurf® A004-UP and Silsurf® C208; silicone glycol copolymers, such asSilsurf® C208; and fluorinated silicones, such as Fluorosil® 2110; allsupplied by Siltech Corporation) may be preferable over less polar ornon-polar counterparts of the same families, but having a lower amountof polar moieties (e.g., silicone glycol copolymers, such as DMS-T21;and fluorinated silicones, such as DMS-T22; all supplied by Gelest®).Polar lubricants (e.g., ethylene glycol, propylene glycol and polymersof the same) may display a lubrication potency of intermediate degreebetween less potent non-polar silicone oils and more potent semi-polarsilicone oils. In some embodiments, water may also serve as a polarlubricant.

In some embodiments, lubricant 730 may be a non-swelling liquid withrespect to the surfaces that lubricant 730 may contact in operation. Forinstance, a non-swelling lubricant does not significantly modify theshape and/or the weight of a support layer 710, a compressible element708, a compressible segment 92 of a pressure applicator 90, and/or asegment 94, as the case may be. The ability of a liquid (e.g., alubricant) to swell any part or material can be assessed by dipping thepart or material under study in the candidate liquid for a predeterminedduration (e.g., 24 hours) and by measuring (after having wiped excessliquid) the difference in weight, if any, before and after incubation ofthe tested part or material in the candidate liquid. A liquid yieldingsubstantially no change in weight of the incubated part or material isconsidered non-swelling with respect to said part or material. A weightafter incubation in a liquid having a deviation from the original weight(before incubation) within ±5% is considered substantially identical tothe original weight, the liquid being therefore deemed “non-swelling”.In some embodiments, the deviation from original weight after 24 hoursof incubation with the non-swelling lubricant is 4% or less, 3% or less,2% or less, or 1% or less. Swelling, or lack thereof, may be similarlyassessed by changes in volume of the tested part or material in thecandidate liquid. The ability of a lubricant to swell or not swell aparticular part or material it may contact in the printing process canbe assessed at the intended operating temperature.

It is to be noted that a lubricant having a relatively low molecularweight can more easily penetrate a polymer matrix than a lubricanthaving a relatively high molecular weight, a low molecular weightlubricant having therefore increased probability of swelling a matrix.However, molecular weight is not the sole parameter indicative of theability of a lubricant to swell or not swell a particular material bypenetrating underneath its surface and the relative polarity of thecontacted matters may also play a role. Non-polar lubricants may swellsurfaces deemed non swellable by semi-polar or polar lubricants. Withoutwishing to be bound by any particular theory, the Inventors haveobserved that the ability to swell transfer member 700 is inverselyproportional to the polarity of the lubricant.

In some embodiments, lubricant 730 may additionally or alternatively bea non-wetting liquid with respect to the surfaces that lubricant 730 maycontact in operation. For instance, a non-wetting lubricant does notspread on a support layer 710, a compressible element 708, acompressible segment 92 of a pressure applicator 90, and/or a segment94, as the case may be, but bead thereon. The ability of a liquid (e.g.,a lubricant) to wet any part or material can be assessed by depositing adroplet of the candidate liquid on the surface of the part or materialunder study and observing the behavior of the droplet. This ability canbe further assessed by measuring the contact angle formed by the dropleton the surface. If desired, the tendency of a liquid to wet, or not wet,a particular surface can also be estimated by measuring the respectivesurface tension and surface energy of the materials contacting oneanother. A liquid beading on a surface of a part or material isconsidered non-wetting with respect to said part or material.

In any configuration of the printing system, lubricant 730 isadvantageously transparent, and has the same or similar refractive indexas the transparent transfer member 700 and/or the refractive index ofthe compressible element 708 of an imaging station 16 or of segment 94of an imaging station 16′, to minimize reflections at the interfacebetween the compressible element (or any other element in the opticalpath) and the transfer member.

According to some embodiments lubricant 730 is selected to be atransparent glycol or polyether, selected from a group comprisingethylene glycol (EG), polyethylene glycol (PEG), propylene glycol (PG)and polypropylene glycol (PPG). An EG, PEG, PG or PPG transparentlubricant is inert with respect to the polymeric matrix of thecompressible element 708, or of any other element it may contact (e.g.,segment 92 and/or segment 94), and/or of the transfer member 700 (inparticular, with respect to the layer on the rear side of the transfermember, e.g., support layer 710). By inert, it is meant in the presentcontext, that the lubricant is non-swelling and non-wetting of thesurfaces that the lubricant may contact in operation. In someembodiments, the EG, PEG, PG or PPG lubricant has a relatively lowviscosity. In such embodiments, the EG, PEG, PG or PPG lubricant mayhave an average molecular weight of 600 or less, 500 or less, or 400 orless. EG, PEG, PG or PPG, when used as lubricant 730 according to someembodiments of the present teachings may have an average molecularweight of 40 or more, 60 or more, or 80 or more. In some embodiments,the lubricant 730 is ethylene glycol (EG) or a PEG polymer selected froma group comprising PEG-200, PEG-300, and PEG-400. In some embodiments,lubricant 730 is propylene glycol (PG) or a PPG polymer selected from agroup comprising PPG-Mn˜425, PPG-P400, PPG-P1200, and PPG-2000.

According to some embodiments lubricant 730 is selected to be atransparent silicone oil compatible with the silicone matrix of thecompressible element 708 and/or the silicone matrix of the transfermember 700. By compatible, it is meant in the present context that thelubricant, while being non-wetting, may to some extent swell the matrixof the compressible element 708 and/or of the transfer member 700, so asto achieve, if desired, a particular effect to be detailed in thefollowing. The lubricant typically has a surface tension higher than thesurface energy of the transfer member and different than the surfaceenergy of the compressible element. The lubricant, if and when sweatingout of a silicone matrix should preferably bead on the surface of atleast one of the two surfaces the oil is due to lubricate.

According to some embodiments, the silicone oil is further adapted topenetrate through the transfer member so as to replenish the content ofsilicone oils that may exude on the imaging surface during operation.Without wishing to be bound by any particular theory, it is believedthat the oil constituents that may be released from a cured matrix withtime can form a thin film upon the imaging surface and enhance releaseof tacky particles or film onto the substrate at the transfer station asdescribed above. In such embodiments the use of a suitable silicone oilas a lubricant on the rear side of the transfer member may prolong theuseful life expectancy of the transfer member, because, in contrast tospontaneous release of silicone oils from a silicone matrix, which maydiminish and even end over time, the added lubricant is suppliedincessantly during operation. The viscosity of the silicone oil may beselected in accordance with the permeability of the silicone matrix ofthe transfer member and with the total thickness thereof, to obtainsufficient penetration of the silicone oil through the transfer member,yet to avoid swelling of the transfer member to an extent that mayaffect the imaging surface uniformity, hence print quality. Similarly,the molecular weight of the silicone oil may be small enough to allowdiffusion through the transfer member, yet sufficiently high to controlthe rate of diffusion. Generally, the amount of silicone oil that may bedesorbed from an elastomeric matrix is sufficiently high to provide thedesired release of the image, yet sufficiently low, so as to avoid anysignificant transfer to the printing substrate. A lubricant, which in aparticular embodiment, facilitates the release of the ink image from thetransfer member to the printing substrate is considered “a releaseenhancing aid”.

The silicone oil that may serve as transparent lubricant can berelatively polar as compared to the silicone matrix to be “replenished”therewith. The transparent lubricant can be selected from polyethersilicone oils and amino-silicone oils. Non-limiting examples of suitablesilicone oils include Silsurf® A004-UP and Silsurf® C208, polyethersilicones commercialized by Siltech Corporation, and GP-4, anamino-silicone oil commercialized by Genesee Polymers Corporation.

The digital printing system shown in FIGS. 3a and 4 can only print inone color but multicolor printing can be achieved by passing the samesubstrate successively through multiple arrangements of printingsystems, such as those herein disclosed consisting of coating, imagingand transfer stations, that are synchronized and/or in registration withone another and each printing a different color. Such a printingassembly, including at least one printing system according to thepresent teachings, is schematically depicted in FIG. 9. In suchembodiments it may be desirable to provide substrate treating stationsbetween the different coating stations (such a treating station 900 isschematically represented in FIG. 9). A treating station can be, forinstance, a cooler able to reduce the temperature of the substrate onits exit of a previous transfer station. While FIG. 9 illustrates anarrangement including two printing systems 1000 a and 1000 b comprisingcoating stations (14′ and 14″), imaging stations (16′ and 16″) andtransfer stations (18′ and 18″), this should not be construed aslimiting, and a printing system may include any other number of suchprinting sub-arrangements (e.g., 3, 4, 5, and so on). When considering aprinting system being an arrangement of a series of printing systemssub-arrangements, printing on a same substrate, the most upstreamtransfer station can be referred to as the first transfer station andthe most downstream transfer station can be referred to as the lasttransfer station, which can be a second, third, fourth and so ontransfer station. Such arrangement can be alternatively referred to as aprinting assembly, each transfer station being of a distinct printingsystem. While the pair of printing systems illustrated in the printingassembly of FIG. 9 relates to a printing system corresponding to FIG. 3a, the same could be achieved by using a printing system according toFIG. 4 or 8, or combinations of such printing systems as diversesub-arrangements in a same printing assembly.

As some transferred films may retain some residual tackiness to a degreethat may impair a subsequent transfer of different particles, it may beadvantageous to eliminate such residual tackiness by cooling of the filmtransferred to the substrate. Depending on the thermoplastic polymer,the elimination of any residual tackiness, or its reduction to a levelnot affecting the process, can alternatively be achieved by a treatingstation being a curing station. This, however, is not essential asretaining tackiness may be desirable for alternative printing assembliesin which a printing system downstream of one according to the presentteachings is different, as schematically illustrated in FIGS. 6a and 6b.

Moreover, while in previous paragraphs each printing system (arrangementof coating, imaging and transfer stations) was considered for the sakeof printing a different color of thermoplastic particles, in a furtherembodiment, one set of such printing systems in a printing assemblycomprising at least two such printing systems, can be used to applycolorless particles. For instance, the colorless particles can beapplied at the final printing system in the assembly. In such a case,the colorless film of tacky thermoplastic particles of the last coatingstation, exposed to radiation at the last imaging station, aretransferred at the transfer station of the last printing system, forinstance, to serve as overcoat to the previous colored films. Such adownstream printing system can be said to form an over-coatingarrangement or sub-system. Conversely, an arrangement for colorlessprinting can be the first printing system of a printing assembly, forinstance, to modify the later application of colored particles or films,and/or the visual effect they may provide. Such an upstream printingsystem can be said to form an under-coating arrangement or sub-system.

Furthermore, a printing system, even if monochrome, may include aperfecting system allowing double-sided printing. In some cases,perfecting can be addressed at the level of the substrate transportsystem 200, which may for example revert a substrate to a side not yetprinted on and return the unprinted side of the substrate to the sametreating and impressions stations having served to print the first side.In other cases, perfecting can be addressed by including two separatetransfer stations (and their respective upstream or downstreamstations), each transfer station enabling printing on a different sideof the same substrate.

FIG. 6a shows an embodiment in which a first printing system, generallydesignated 1000, is the same as that illustrated in FIG. 3a . The samereference numerals have been retained to avoid the need for repetition.The first printing system can alternatively be as schematicallyillustrated in FIGS. 1 and 4. The first transfer station at which thethermoplastic particles can be transferred to the substrate to form anadhesive image is designated 218. In one embodiment, a second printingsystem 2000 is provided to coat the substrate, or selected regionsthereof, with second particles of any desired material and color, whichcan serve as a varnish or protective or decorative coat. In thisprinting system 2000, a coating station 14′ applies a monolayer ofsecond (e.g., transparent) particles to a transfer member 700′ passingover a drum 30′. There is however no selective heating of the particlesin the transfer member 700′. Instead, the transfer member 700′ ispressed against the substrate 20 conveyed by at substrate transportsystem 200, and second (e.g., plastic, metallic or ceramic) particlesare transferred to the substrate at transfer station 228′ either becausethe polymer film on the substrate applied by the printing system 1000 isstill tacky (i.e. sufficiently adhesive to transfer second particlesfrom their transfer member and adhere them to the substrate), or becausea pressure roller 720 and/or the impression cylinder 22′ is heated, saidheating restoring the tackiness of the image transferred at the firstprinting system and/or non-selectively heating the second particles. Forsimplicity, an image formed at a first printing system 1000 whose roleis to adhere second particles brought into contact therewith at a secondprinting system (e.g., 2000, 3000) may be referred to as an adhesiveimage, regardless of any optional need for additional heat once on theprinting substrate for it to be sufficiently tacky/adhesive towards thenew particles. In the former case, only adhesive image areas of thesubstrate will have a coating of second particles (e.g., a varnishcoating if the second particles are thermoplastic), whereas in thelatter case the entire surface of the substrate may receive atransparent coating using such thermoplastic second particles, ifnon-selectively heated to become sufficiently tacky for transfer.

As previously described for the first printing system wherein theimaging surface 12 can be replenished with thermoplastic particles, theparticle receiving surface 712′ of the second printing system cansimilarly be replenished with second particles at the second coatingstation 14′ following transfer of at least part of them to the substrateat a previous cycle. In such way, the particle receiving surface willagain be uniformly coated with a monolayer of second particles beforecoming into contact with a subsequent substrate and adhesive imagethereon. While the second particles are typically applied to thereceiving surface (e.g., 712′, 712″) so as entirely coat it, in someembodiments the density of the particles coating the second transfermember 700′ or 700″ may be controlled by coating station 14′ so as tocoat only portions of the adhesive image created by the first printingsystem 1000. For illustration, if metallic particles are applied on thereceiving surface of a second transfer member at a relatively lowdensity achieving e.g., 5% area coverage, transfer of said relativelydispersed second particles to an adhesive image may provide for aglittering effect to the previously applied image.

Alternatively, the second printing system can be set to print on onlypart of (e.g., the width of) the substrate, so that even an entirelycoated receiving surface of second particles may transfer to only a partof the adhesive image. For illustration, if the particle receivingsurface of the second printing system is offset with respect to theimaging surface of the first printing system or with respect to anadhesive image that can be formed thereby, then the second particles mayadhere to only part of the adhesive image, such as in a bottom, centralor top portion thereof. A similar effect can alternatively be obtainedby selectively heating only a part of the adhesive image to achieve anadhesiveness suitable to detach second particles. For illustration, aheating station upstream of the nip of a second printing system may beshaped or adapted to heat selective areas of the substrate and of anadhesive image formed thereon, yielding for instance stripes of heatedareas. In any such event, the second particles are said to adhere to atleast a part of the adhesive image.

Particles rendered tacky at printing system 1000, and polymer filmsthereof subsequently transferred to the substrate, may retain at leastsome degree of tackiness from the time they are applied on the substratetill it reaches the nip of printing system 2000. This can be achieved byeither ensuring that the thermal characteristics of the transfer memberand/or the particles are adequate to keep the particles warm enough(i.e. tacky) until said contact is made, or, preferably, by employingthermoplastic particles which have a delayed crystallizationcharacteristic (called “open time” in hot melt adhesive parlance)adequate to retain tackiness and adhesiveness until pressed into contactwith the substrate and/or until reaching a second printing system.

FIG. 6a also shows a finishing station 746 where the polymer film mayundergo (e.g., thermal) treatment to fix, cure, coat or dry the polymerfilm applied by printing system 1000 and/or by printing system 2000. Ifsuch (e.g., thermal) treatment is accompanied by pressure contact withthe polymer film, it may also serve to impart a desired surface finish,such as a gloss, to the surface of the substrate.

In FIG. 6b , the first printing system 1000 may be as previouslydescribed for FIG. 6a , this figure illustrating an alternative printingassembly wherein the second printing system is designated 3000. While inFIG. 6a the second particles applied at the second printing system 2000,were applied to the particle receiving surface 712′ of a second transfermember 700′ shaped as an endless belt, in the printing assembly of FIG.6b the second particles are applied by the second coating station 14′ tothe particle receiving surface 712″ of a second transfer member 700″shaped as a drum. Drum 700″ not only provides the receiving surface tobe coated by the second particles, but forms with the impressioncylinder 22′ the second transfer station 228″ and the nip through whicha substrate 20 would pass upon exit of the nip of the first transferstation 218 of the first printing system 1000. FIG. 6b illustrates afurther alternative for a printing assembly, wherein the adhesive imageformed by selective transfer of thermoplastic particles by the firstprinting system 1000 can be optionally subsequently heated at a heatingstation 750. Heater of heating station 750 may ensure that the adhesiveimage remains sufficiently tacky (or regains sufficient tackiness) bythe time the substrate enters the second nip. While schematicallyrepresented by a box upstream of printing system 3000, the heatingelements can additionally or alternatively be positioned at the nip ofthe second transfer station, impression cylinder 22′ containing forillustration an internal heating element or circulating liquid.Regardless of position at which it is directed along the printing path,the applied heat can be selected to render tacky only the adhesive imageor portions thereof. In some embodiments, when complete coating of thesubstrate is not sought, the heat can be configured to be sufficientlylow so as to prevent non-selective transfer of thermoplastic secondparticles to the entire surface of the substrate. Generally, secondparticles made of materials other than thermoplastic polymers, such asthermosetting plastics, metals, ceramics, glasses and the like, transferonly to the areas of the adhesive images or to selectively heatedpart(s) thereof.

Elements of the exemplary embodiments of printing assemblies, andsub-systems thereof, separately illustrated in either FIG. 6a or FIG. 6bmay be combined in any desired manner. For illustration, a heatingstation 750 can be present upstream and/or at a second transfer station,wherein the second particles are applied to a second transfer membershaped as an endless belt (e.g., 700′). It should be noted that as thesecond printing systems illustrated in these figures do not necessarilyrely on EM radiation or heat transiting via a rear side of the transfermember to effect transfer of second particles, which moreover would notnecessarily be thermoplastic, their transfer members (e.g., 700′, 700″)need not be transparent to EM radiation. Therefore, while secondtransfer members of second printing systems can be configured asextensively detailed for transfer member 700, they may be subject toless stringent requirements, as long as they are capable of transientlyretaining the second particles and being able to release them oncecontacted by an adhesive image.

The Substrate

The printing systems and assemblies shown in FIGS. 1, 3 a, 4, 6 a, 6 b,8 and 9 are not restricted to any particular type of substrate. Thesubstrate may be individual sheets of paper or card or it may have theform of a continuous web. Because of the manner in which a thin film ofsoftened polymeric particles is applied to the substrate, the film tendsto reside on the surface of the substrate. This allows printing of highquality to be achieved on paper of indifferent quality. Furthermore, thematerial of the substrate need not be fibrous and may instead be anytype of surface, for example a plastics film or a rigid board.

The Transfer Station

The transfer station 18, 18′ or 18″ illustrated in FIG. 1, 3 a, 4, 8 or9, or the transfer stations 218, 228′ and 228″ illustrated in FIGS.6a-6c , have been depicted to comprise only a smooth impression cylinder22 or 22′ that is pressed against the drum 10, or the transfer member700, 700′ or 700″, and their outer imaging surface 12 or particlereceiving surfaces 712′ or 712″. The impression cylinder 22 and/or 22′may form part of a substrate transport system 200, in which case it maybe equipped with grippers for engaging the leading edge of individualsubstrate sheets. In other than digital printing systems, the impressioncylinder 22 may have an embossed surface to select the regions of theparticle coating to be transferred to the substrate 20.

Additional Stations and/or Transfer Members

The printing systems described herein may have additional stationsand/or transfer members. For example, a particular printing assembly mayfurther include additional printing systems which may include a secondcoating station, a second transfer station, and a second transfermember. The second transfer station (e.g., 18″ of FIG. 9) may bedisposed downstream of the afore-described (first) transfer station(e.g., 18′ of FIG. 9) in the path of the substrate. The second coatingstation (e.g., 14″ of FIG. 9) may serve to apply a monolayer coating ofdifferent polymeric particles to the second transfer member (e.g., 700″of FIG. 9) that is pressed against the substrate at the second transferstation. For instance, the particles that are applied to the secondtransfer member may be of a second color or may be transparent.Additionally, or alternatively, the particles applied to the secondtransfer member may adhere only to regions of the surface of thesubstrate having a polymer film that was applied at the (first) transferstation and that is still tacky. In embodiments with such additionaltransfer stations, the finishing station discussed above may apply athermal treatment to the polymer film that was applied to the substrateat the last of the transfer stations (e.g., after passage through allthe transfer stations). While the different transfer stations in aprinting assembly comprising more than one printing system have beenillustrated in FIGS. 6a-6b and in FIG. 9 as being separately formed withdistinct impression cylinders, such arrangement is not mandatory. In analternative embodiment of a printing assembly, the transfer stations ofa plurality of printing systems can be arranged across a same impressioncylinder. For illustration, a printing assembly comprising four printingsystems, each adapted to thermally transfer thermoplastic particles of adifferent color, such as cyan, magenta, yellow and black, can beradially positioned along the circumference of a single impressioncylinder. This is schematically depicted in FIG. 6c , in which each of1000 a to 1000 d can be adapted to transfer thermoplastic particles of adifferent color. Alternatively, one of 1000 b to 1000 d can serve as asecond printing system (such as detailed for 2000 and 3000) capable ofapplying second particles to an adhesive image formed by 1000 a being afirst printing system. For sake of clarity, in-between or finishingstations and optional heaters(s) are omitted from this figure.

In the description and claims of the present disclosure, each of theverbs, “comprise” “include” and “have”, and conjugates thereof, are usedto indicate that the object or objects of the verb are not necessarily acomplete listing of features, members, components, elements, steps orparts of the subject or subjects of the verb.

As used herein, the singular form “a”, “an” and “the” include pluralreferences and mean “at least one” or “one or more” unless the contextclearly dictates otherwise. At least one of A and B is intended to meaneither A or B, and may mean, in some embodiments, A and B.

Positional or motional terms such as “upper”, “lower”, “right”, “left”,“bottom”, “below”, “lowered”, “low”, “top”, “above”, “elevated”, “high”,“vertical”, “horizontal”, “front”, “back”, “backward”, “forward”,“upstream” and “downstream”, as well as grammatical variations thereof,may be used herein for exemplary purposes only, to illustrate therelative positioning, placement or displacement of certain components,to indicate a first and a second component in present illustrations orto do both. Such terms do not necessarily indicate that, for example, a“bottom” component is below a “top” component, as such directions,components or both may be flipped, rotated, moved in space, placed in adiagonal orientation or position, placed horizontally or vertically, orsimilarly modified.

Unless otherwise stated, the use of the expression “and/or” between thelast two members of a list of options for selection indicates that aselection of one or more of the listed options is appropriate and may bemade.

The word “exemplary” is used herein to mean “serving as an example,instance or illustration”. Any embodiment described as “exemplary” isnot necessarily to be construed as preferred or advantageous over otherembodiments and/or to exclude the incorporation of features from otherembodiments.

Unless otherwise stated, when the outer bounds of a range with respectto a feature of an embodiment of the present technology are noted in thedisclosure, it should be understood that in the embodiment, the possiblevalues of the feature may include the noted outer bounds as well asvalues in between the noted outer bounds.

In the disclosure, unless otherwise stated, adjectives such as“substantially”, “approximately” and “about” that modify a condition orrelationship characteristic of a feature or features of an embodiment ofthe present technology, are to be understood to mean that the conditionor characteristic is defined to within tolerances that are acceptablefor operation of the embodiment for an application for which it isintended, or within variations expected from the measurement beingperformed and/or from the measuring instrument being used. When the term“about” or “approximately” precedes a numerical value, it is intended toindicate +/−15%, or +/−10%, or even only +/−5%, and in some instancesthe precise value. Furthermore, unless otherwise stated, the terms(e.g., numbers) used in an embodiment of the present technology, evenwithout such adjectives, should be construed as having tolerances whichmay depart from the precise meaning of the relevant term but wouldenable the embodiment or a relevant portion thereof to operate andfunction as described, and/or as understood by a person skilled in theart.

While this disclosure has been described in terms of certain embodimentsand generally associated methods, alterations and permutations of theembodiments and methods will be apparent to those skilled in the art.The present disclosure is to be understood as not limited by thespecific embodiments described herein.

To the extent necessary to understand or complete the disclosure of thepresent invention, all publications, patents, and patent applicationsmentioned herein, including in particular the applications of theApplicant, are expressly incorporated by reference in their entirety byreference as if fully set forth herein.

Certain marks referenced herein may be common law or registeredtrademarks of third parties. Use of these marks is by way of example andshall not be construed as descriptive or limit the scope of thisdisclosure to material associated only with such marks.

1. A printing assembly comprising: A] a first printing systemoperational to effect printing onto a surface of a substrate by thermaltransfer, the first printing system comprising: a) a transfer memberhaving opposite front and rear sides with an imaging surface on thefront side; b) a coating station at which a monolayer of first particlesmade of, or coated with, a thermoplastic polymer is applied to theimaging surface, or at least a segment thereof, the transfer member andthe coating station being operationally in relative movement; c) animaging station at which energy in the form of electromagnetic (EM)radiation is applied to selected regions of the imaging surface torender first particles coating the selected regions tacky; and d) atransfer station at which the imaging surface of the transfer member andthe surface of the substrate, or respective segments thereof, areoperationally pressed against each other to cause the first particlesthat have been rendered tacky to transfer to the surface of thesubstrate whereby an adhesive image is formed on the substrate; whereinthe transfer member is formed of a body on the rear side and an EMradiation absorbing layer made of an elastomeric silicone on the frontside of the transfer member adjoining the body, the imaging surfacebeing formed on, or as part of, the radiation absorbing layer; and B] asecond printing system configured to apply second particles; theprinting assembly further comprising a heating station at which at leasta part of the adhesive image on the substrate is heated so that it istacky at least during the application of the second particles at thesecond printing system, to cause second particles to adhere to the atleast heated part of the adhesive image.
 2. The printing assembly ofclaim 1, wherein the EM radiation applied to the selected regions of theimaging surface is applied to the front side of the transfer member. 3.The printing assembly of claim 1, wherein the EM radiation applied tothe selected regions of the imaging surface is applied via the rear sideof the transfer member, the body of the transfer member beingtransparent to the EM radiation.
 4. The printing assembly of claim 3,further comprising a lubrication system configured to controllablyrelease a lubricant to the rear side of the transfer member of the firstprinting system.
 5. The printing assembly of claim 3, wherein thetransfer station of the first printing system comprises an impressioncylinder positioned facing the front side of the transfer member so asto define a nip at which at least a segment of the imaging surface ofthe transfer member and at least a segment of the surface of thesubstrate are pressed against each other, and wherein the imagingstation is configured and aligned to apply radiation to the rear side ofthe transfer member at and/or adjacent the nip, so that rendering of thefirst particles coating the selected regions tacky, and pressing of theimaging surface of the transfer member and the surface of the substrate,or respective segments thereof, against each other, occur substantiallyconcurrently.
 6. The printing assembly of claim 5, wherein the imagingstation further comprises a transparent member facing the rear side ofthe transfer member at the nip, the transfer member sliding duringoperation between the transparent member and the impression cylinder,the applied EM radiation passing through the transparent member.
 7. Theprinting assembly of claim 6, wherein the transparent member is one of:a) a compressible transparent member; b) a non-compressible transparentmember, the first printing system further comprising a pressureapplicator having a compressible segment in contact with the rear sideof the transfer member adjacently to the nip; c) a transparent memberincluding a surface disposed to contact at least in a portion thereof, aportion of the rear side of the transfer member, the surface having oneor two rounded edges to facilitate for a lubricant at least one of entryin-between the surface and the rear side and exit from in-between thesurface and the rear side; and d) a transparent member including asurface disposed to contact at least in a portion thereof the rear sideof the transfer member, the surface having one or more grooves fordirecting traversal of a lubricant between the surface and the rearside.
 8. The printing assembly of claim 1, wherein the coating stationis configured to apply a fresh monolayer coating of first particles tothe selected regions from which the first particles were previouslytransferred to the substrate surface to form the adhesive image, torender the imaging surface uniformly coated with a monolayer of firstparticles.
 9. The printing assembly of claim 1, wherein the imagingsurface is the outer surface of a drum or of an endless transfer member.10. The printing assembly of claim 1, wherein the second printing systemcomprises: a) a second transfer member having opposite front and rearsides with a particle receiving surface on the front side; b) a secondcoating station at which a monolayer of second particles is applied tothe particle receiving surface, the second transfer member and thesecond coating station being operationally in relative movement; and c)a second transfer station at which the particle receiving surface of thesecond transfer member and the surface of the substrate, or respectivesegments thereof, are operationally pressed against each other to causetransfer of the second particles to at least a part of the adhesiveimage formed on the substrate at the first printing system.
 11. Theprinting assembly of claim 10, wherein the second transfer station ofthe second printing system comprises a second impression cylinder facingthe front side of the second transfer member, the second impressioncylinder being positioned so as to define a second nip at which theparticle receiving surface of the second transfer member and the surfaceof the substrate, or respective segments thereof, are pressed againsteach other, the second impression cylinder being same or different thanthe impression cylinder of the first printing system.
 12. The printingassembly of claim 11, wherein the heating station is disposed at oradjacent to the second nip, the heat being applied to at least a part ofthe adhesive image at and/or adjacent the second nip, so that heatingthe adhesive image and pressing of the particle receiving surface of thesecond transfer member and the surface of the substrate, or respectivesegments thereof, against each other, occur substantially concurrently.13. The printing assembly of claim 11, wherein the particle receivingsurface of the second transfer member is the outer surface of a) asecond drum and the second nip is between the second drum and the secondimpression cylinder, or b) an endless second transfer member and thesecond nip is between a pressure roller facing the rear side of theendless second transfer member and the second impression cylinder. 14.The printing assembly of claim 13, wherein the second impressioncylinder is different than the impression cylinder of the transferstation of the first printing system, and at least one of the drum,pressure roller and second impression cylinder is capable of heating soas to constitute at least a part of the heating station.
 15. Theprinting assembly of claim 13, wherein at least one of the drum,pressure roller, impression cylinder and second impression cylinderfurther includes a compressible layer on its outer surface.
 16. Theprinting assembly of claim 10, wherein the second coating station isconfigured to apply a fresh monolayer coating of second particles toselected regions of the particle receiving surface depleted from secondparticles previously transferred to at least a part of the adhesiveimage, to render the particle receiving surface uniformly coated with amonolayer of second particles.
 17. The printing assembly of claim 1,comprising at least one of: a) an under-coating arrangement locatedupstream of the first printing system; b) a cooling station locateddownstream of the second printing system; c) an over-coating arrangementlocated downstream of the second printing system; and d) a finishingstation located downstream of at least one of the printing systems. 18.The printing assembly of claim 1, wherein a) the first and secondparticles are colored particles, a color of the first particles beingdifferent from a color of the second particles; orb) one of the firstand second particles are colored and the other of the first and secondparticles are transparent.
 19. The printing assembly of claim 1, whereinthe second particles provide a decorative coating to the adhesive image,the second particles being made of plastics, metals, ceramics orglasses.
 20. The printing assembly of claim 1, further comprising acooler upstream or on the entry side of the coating station and/or aheater on the exit side of the coating station.